In the bowl of a stand mixer, cream together the butter and sugar until well blended and fluffy (like 2 minutes).

Add the eggs and beat until well-incorporated, scraping down the sides, then beat in the cocoa powder.

Mix in the salt, baking powder, and 2 cups of the flour (with cornstarch, if applicable) until just combined and then add however much of the remaining flour you need. The dough should be thick, dense, and drier than most cookie dough, but not actually dry or really crumbly (see picture in the post for what my dough looked like).

Transfer the dough to a clean work surface or giant bowl and gently fold in the chocolate chips and peanut butter chips by hand, gently mixing with your hands to ensure even distribution.  Don't overwork it, or the cookies will be tough.

Divide into 4-ounce (~110-120g) portions (easiest with a kitchen scale, otherwise just make...giant balls of dough), and place on a sheet pan lined with parchment paper.  I found an ice cream scoop was really helpful here, it's *just* larger than what you need.

Freeze the dough balls for around an hour---YES this is important.

Preheat oven to 350 F.  Bake for 16-20 minutes depending on how gooey and raw you like the middles, taking care not to over-bake (remember they'll solidify once you remove them from the oven).

Let cool on a rack and store what you don't immediately eat in an airtight container.  These are best the next day, just FYI.