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.