• Combine milk, sugar and yeast in the bowl of a stand mixer fitted with a dough hook. Set aside to prove while you prepare the other ingredients.
• Whisk together the flour and salt in a separate bowl. In another bowl, mix together the egg, honey, oil and vanilla.
• Once your yeast is bubbly, pour in your wet and dry ingredients.
• Knead with the hook until the dough comes together in a soft, sticky ball. You may have to add a bit more flour, but avoid adding more than 1/2 of a cup.
• Place dough in an oiled bowl, turn with the spatula or your hands a few times until the dough is completely covered in oil. Cover bowl with plastic wrap and let rise for about 2 hours, until doubled in size (Depending on how warm your kitchen is this could take less or more time.) Alternatively (but recommended) place the dough in the fridge for an overnight rise. It will be easier to work with the next day.

Once the dough is ready, make your filling:

• In the bowl of your stand mixer fitted with a whisk attachment, beat your eggs, sugars and vanilla together until very light and fluffy – about 8-10 minutes.
• Brown the butter: warm butter in a saucepan on low and once completely melted, turn heat to medium high. Let it sputter but keep a watchful eye, once it goes silent, it’s done. Place it in a heatproof bowl and add the chocolate to it. Stir until the chocolate is completely melted.
• Whisk together the cocoa, salt, and flour in a separate bowl.
• When the eggs are done whipping, add the warm chocolate mixture and stir to combine. Add the flour and turn the mixer on with the whisk attachment until the dry ingredients are just combined. Use a spatula to scrape bottom and sides of bowl to incorporate.

Make the babka:

• Roll out your challah dough into a rectangle on a well-floured surface. The shorter side should not be more than 1-2 inches longer than your loaf pan.
• Spread your brownie batter over the dough, leaving a half inch border on all sides.
• Roll the dough up into a log from the shorter side (so, your log will be about as long as your loaf pan). Slice with a knife down the middle lengthwise and twist the two logs around each other.
• Prepare a standard loaf pan with oil and parchment paper. Place the twisted dough in the pan and set in a warm place to rise for 45 minutes. If you have done the overnight rise, it’ll need a longer second rise, probably closer to 1.5 hours.
• Preheat oven to 375 F. Mix the leftover egg white with some water and gently brush it over the challah dough (avoid the brownie).Cover with foil and bake for 25 minutes. Remove the foil and bake for another 5-15 minutes. The bread is done when a thermometer inserted into the middle of the dough reads 190 F.