Filed under: Bars / Brownies & Blondies
March 17, 2021

Gooey Rainbow Bars

Gooey rainbow marshmallow bars sit atop a layer of oreos and a cream cheese shortbread. A super easy bake, an impressive and super sweet result.

5 from 3 votes
Jump to recipe

Gooey rainbow marshmallow bars sit atop a layer of oreos and a funfetti shortbread. A super easy bake, an impressive and super sweet result.

Sometime ago the pinterest baking community got obsessed with ‘slutty’ brownie; bars of three parts: chocolate chip cookie on the bottom, oreos in the middle and brownie on top. My oreo stuffed two toned cookies were made with that recipe in mind. I later learned that there is a reese’s version of these too (swap or add pb cups in the oreo layer). These gooey rainbow bars are inspired by what’s called ‘slutty brownies’ but swap out the bottom for a tangy cream cheese shortbread and the top for something altogether new. I call it a blondie for familiarity’s sake but if you don’t use brown sugar they aren’t technically blondies. 

Let’s talk layers: the bottom is a crumbly and buttery, a note of dark cocoa comes in through the oreos (the buttercream sandwiched in there makes these bars four layers!) and on top is my absolute favorite part, oozy gooey gorgeous marshmallow bars. 


Gooey Rainbow Marshmallow Bars

This is the layer I’ve been working on and wanting to share here for a while. I LOVE IT so much! It’s a blondie but only if you use brown sugar. The first couple of times I made these I made them with granulated sugar and by themselves (no oreo, no shortbread – pics below!) and called them gooey rainbow bars because really they are exactly that. I felt like they needed something, a better vehicle to be served with so I added the other two layers on a whim, and it worked! 


Funfetti Shortbread 

This is a really easy layer, you’ll just beat everything together and then press it into the pan in one layer. You can use regular jimmy sprinkles (the longer thin shape) or non-pareils (the little balls). Since there’s isn’t a lot of liquid in this recipe either shape will work fine here. 

A note on sweetness! 

Somebody is going to say, these are too sweet! Yes, yes they are. Be prepared. If you are making a recipe that calls for oreos, blondies, sprinkles AND marshmallows… you best bet it’s going to be a very sweet bar. Don’t make them if you have a shy sweet tooth. Or make them and give them to someone who can really appreciate them. 

What substitutions can I make? 

  • Oreos for golden oreos. Or oreos for pb cups! 
  • Lucky charms marshmallows for more sprinkles (but use a lot less, probably ¼ cup)
  • Cake flour for all purpose flour: this will make the bottom layer softer, the top layer gooier. 

Tips & what to get right: 

  1. Have the butter & cream cheese for the bottom layer at room temperature so they blend easily into the sugar. 
  2. Be ready with your blondie batter when the shortbread is done par-baking. 
  3. To get that lovely shiny top you need to melt some of the sugar into the butter, follow the steps closely. 



Gooey Rainbow Bars 

5 from 3 votes


Funfetti Shortbread

  • ½ cup unsalted butter softened (113g)
  • ½ cup sugar 100g
  • ½ teaspoon fine sea salt
  • 1 teaspoon imitation vanilla or pure extract
  • 4 oz cream cheese half the 8 oz package
  • 1 1/2 cups flour 188g
  • 2 tablespoons sprinkles

15 chocolate sandwich cookies (optional)

  • Gooey Rainbow Marshmallow Layer
  • 1 stick unsalted butter cold is fine (113g)
  • 1 cup granulated sugar 200g
  • 1 large egg
  • 1 cup all purpose flour 125g
  • ¼ teaspoon fine sea salt
  • 1 teaspoon imitation vanilla or pure extract
  • ¾ cups lucky charms marshmallows


  • Preheat the oven to 350 F. Grease and line an 8x8” square pan with parchment paper.
  • Beat together the cream cheese, butter and sugar until light and fluffy. About 3-5 minutes.
  • Add the flour, salt and vanilla and beat to just combine. Fold in the sprinkles.
  • Press the dough into the bottom of the pan in an even layer. Use the back of a measuring cup to pack it in.
  • Bake for 15 minutes.
  • While the bottom layer bakes, melt the butter in a heatproof bowl in the microwave (about 60 seconds).
  • Stir the sugar into the butter. Return it to the microwave for 30 seconds. The mixture should be hot and part of the liquid butter will sit on top. Whisk it really well, for at least a full minute and it will blend.
  • Add the egg and whisk for another full minute (this is important, to emulsify the egg into the oil). The mixture should be shiny and smooth.
  • Whisk in the flour, salt and vanilla and fold in the charm marshmallows.
  • When the shortbread has baked for 15 minutes, lay the oreos on top as pictured above. Spread the blondie batter over it and smooth it into an even layer (it’s thick so it will take some prodding, use the back of a spoon or an offset spatula).
  • Return to the oven and bake for another 30-35 minutes.

Did you make this recipe?

Share & tag me on instagram @buttermilkbysam


Rate + Review

What do you think of this recipe?

Your email address will not be published. Required fields are marked *

Recipe Reviews

  1. These are so very tasty! I went full sprinkle on this one and didn’t use marshmallows (3 tbsp of sprinkles for the cake layer). The bars definitely have a birthday cake vibe. Thank you for the recipe, Sam!

  2. 5 stars
    Surprisingly enough, I didn’t think these were too sweet (maybe that’s a problem on my end ?) but rather perfectly balanced! The tanginess of the cream cheese in the shortbread layer complimented the sugar-sweet of the rest of the bar. I used chocolate birthday cake flavored oreos because I felt like going all in lol! It was as fun making these as it is to look at the final product ?

  3. 5 stars
    Just made these as the “birthday cake” for my sister-in-law, whom I love. 🙂 I told her that I show my love by picking all the marshmallows out of a box of Lucky Charms! She’d sent me this recipe months ago, and post-Thanksgiving made them for her. They’re definitely sweet, but not cloying – and fun to make! The Birthday Girl loved them, all of our kids adored them, they’re pretty and fun. Not an everyday treat for me, but definitely felt celebratory! We enjoyed them so much! Thanks for yet another winner!

  4. 5 stars
    These are simply amazing!! One of the best things I have made from Sam recipes. The dried marshmallows add that flavor without the gooey mess of regular marshmallows. I would definitely make these again and experiment with different oreo flavors. And they are a great bake to make with kids!

  5. A few thoughts on clarifying the instructions:
    1) If you use room temp butter and cream cheese for the blondie layer, it creates a very sticky dough that is difficult to “pack down” as instructed and almost impossible to smooth over parchment.
    2) The ingredients for the marshmellow layer reference vanilla and salt, but you don’t note when those should be added in the directions.

    We LOVED the recipe regardless and my kiddos had a blast helping me pick out the Lucky Charms marshmellow from the cereal.

    • Emily, I’m so happy you made them and that you guys liked the bars! Fixed the vanilla and salt step, thank you for that. For the top layer, do you mean that the cream cheese should also be soft?

  6. I don’t have lucky charms marshmallows… maybe I can just use mini marshmallows instead?
    What do you think?

    • I used mini marshmallows in my batch and I loved how they turned out! I imagine since Lucky Charms marshmallows are sort of dehydrated, they’ll create less gooeyness than regular mini marshmallows. I’m sure there’s more scientific proof behind that but I don’t know it lol. I think they should taste the same 🙂