For The Most Amazing Brownie Cookie melt the bittersweet chocolate chips and butter in a heavy saucepan over low, stirring constantly until melted and combined. Remove from heat and set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat the eggs, sugars, vanilla, baking powder, and salt on high for 5 minutes, or until the batter is thick and creamy.
Don’t cut this step short! The cookies will be too thin and won’t have the shiny exterior if you cut the mixing short.
Reduce the mixer speed to low and stream in the melted chocolate mixture. Stir in the flour and cocoa powder just until combined.
Stir in the mini chocolate chips and nuts, if using. Cover and chill the dough for 30 minutes.
Scoop out 1.5 tablespoon scoops of dough (medium cookie scoop). Bake for about 8-10 minutes. The middles should look soft and a little underdone but the edges should be set.
Since oven temps can vary, you may find that you need to increase the baking time.