Melt bittersweet chocolate chips 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 fitted with a paddle attachment, beat the eggs, sugars, vanilla, baking powder, and salt on high speed 5 minutes, or until the batter is thick and creamy. This step is key, so make sure to beat the mixture for the full 5 minutes.

Reduce the speed to low, and mix in the melted chocolate until well-combined.

Stir in flour and cocoa powder just until combined.

Add nuts, if using, and mini chocolate chips. Stir in to combine. The batter should be the consistency of a thick brownie batter at this point.