Melt 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
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.
Cover the batter, and chill for 30 minutes
Preheat oven to 350 F. Line two baking sheets with parchment paper.
Using a 1.5 tablespoon cookie scoop, drop batter onto the prepared cookie sheets about 2 inches apart.
Bake cookies 8-10 minutes. The cookie will look set at the edges but still be a little wet looking in the center. Don't overbake, or the cookies won't be crackly and fudgy.
The shiny, crackly crust will develop as the cookies cool on the baking sheet.
I don't recommend Dutch-processed cocoa powder in this recipe.