Melt chocolate and butter in a heavy saucepan over low heat, stirring constantly until melted and well-combined.
Remove from heat, and set aside.
In the bowl of a stand mixer, beat the eggs, sugars, vanilla, baking powder, and salt on high speed 5 minutes, or until the batter is thick and creamy.
Reduce the speed to low, and mix in the melted chocolate until well-combined.
Stir in flour and cocoa powder just until combined.
Add chocolate chips. Stir in to combine.
Cover the batter, and chill while you prepare the chocolate chip cookie dough.
Chocolate Chip Cookie Dough
Melt butter in a heavy saucepan.
Add brown sugar to a large bowl.
Pour melted butter over sugar. Stir to combine butter and sugar. Let stand 5 minutes.
Add egg and egg yolk; stir to combine.
Stir in vanilla extract.
Add flour, baking soda, and salt. Stir until just combined.
Stir in chocolate chips.
Cover, and chill for 30 minutes.
Putting the doughs together
Preheat oven to 350 F. Line 2 large baking sheets with parchment or silicone baking mats.
Using a 1.5 tablespoon cookie scoop (medium), scoop up chocolate chip cookie dough. Repeat with separate scoop for brownie cookie dough (or clean scoop in between).
Press the two doughs together, sealing the seam in the middle.
Repeat with remaining dough.
Bake 9-12 minutes, or until done.
Tips for making brownie cookie dough
Cocoa Powder: I didn’t have good results when using Dutch-processed cocoa powder. I recommend using unsweetened cocoa powder in this recipe.
Chocolate chips: I’ve tried making the cookies without the chocolate chips, and they didn’t have as much of a crackly crust. Don’t skip the chocolate chips if you want a shinier cookies.
Mixing: I recommend using a paddle attachment to mix the cookie dough. I’ve tried using a whisk attachment, and the cookies didn’t turn out as well.
Chilling: The cookies will be flat and will bake out if you skip chilling the dough. Also, if your fridge is packed doesn’t chill very well, you may find that you need to add a little extra time to the given chilling time.
Baking: The cookies will look underbaked in the center but be set at the edges, and the crackly, shiny crust will develop as the cookies cool.
Tips for making chocolate chip cookie dough
Butter: If you want to brown the butter, you will need to use more butter because browning butter causes it to cook down. Otherwise, you will be left with a drier dough. If browning, you will need to use approximately 3 tablespoons more butter (so about 19 tablespoons or 269 grams total).
Correctly measuring the ingredients is key! I suggest either weighing the ingredients or aerating the flour (sifting or fluffing) and then scooping and leveling. If you don't want to sift or weigh the flour, please use less flour (about 2-½ cups). Packing in too much flour will cause the cookies not to spread.
Can substitute 4 ounces finely chopped semisweet baking chocolate for chocolate chips. However, using chopped baking chocolate will cause the dough to spread a little more.
I suggest stirring the dough together by hand rather than using a hand or stand mixer.