Preheat your oven to 200°C / 410°F. Line three baking sheets with baking paper. Set aside.
Add the cocoa powder, all-purpose flour, cake flour, baking soda, cornstarch and salt to a small mixing bowl and use a whisk to combine. Set aside.
Add the butter and sugar into a large mixing bowl. Use an electric hand mixer to cream until pale and fluffy. About 3 minutes. Add the eggs one at a time and mix each time until well combined. You’ll want to scrape down the bowl before you add each egg. Add the dry ingredients and use a wooden spoon or spatula to mix everything until a dough forms and no dry ingredients are showing. Add the peanut butter chips and mix until evenly distributed in the dough.
Use a large ice cream scoop (holds 3 tbsp) to double scoop balls of dough. So all up the dough balls will be 6tbsp in size. You don’t want to shape these into balls, leave them a rough ball shape but flatten slightly before baking. Add 4 cookies on each tray before baking. Bake for 7 minutes. Cookies will be very fudgy and soft when they come out of the oven. Let them cool completely on the tray before serving.