Place the sugar, butter, and cocoa in large, microwave-safe bowl (loosely covered), and microwave in 1-minute intervals until the butter is completely melted.
Stir the mixture together until smooth.
Add the eggs, one at a time, stirring after each addition until completely incorporated.
Stir in the vanilla extract.
Add the flour, baking powder, and salt, stirring together until smooth.
CHOCOLATE CHIP COOKIE DOUGH:
Use an electric hand mixer to combine the butter, brown sugar, and granulated sugar together until very pale and fluffy.
Add the egg, and mix until completely incorporated.
Scrape the bottom and sides of the bowl with a silicone spatula, then add the egg yolk and vanilla, mixing until blended.
Scrape the bottom and sides of the bowl with a silicone spatula, then add the flour, cornstarch, baking soda, and salt.
Mix until all the dry ingredients are incorporated, then stir in the chocolate chips.
ASSEMBLY and BAKING:
Preheat the oven to 375℉, and line baking sheets with parchment paper.
Scoop about 1½ tablespoons of the chocolate chip cookie dough.
Scoop about 1½ tablespoons of the brownie cookie dough.
Press both kinds of dough together, and roll into a ball.
Place on the prepared baking sheets, and bake for 10 to 12 minutes, or until just beginning to turn golden around the edges.