Preheat oven to 400 degrees F. Line two baking sheets with parchment paper.
In a large mixing bowl, cream the butter, granulated sugar, and brown sugar for 3-4 minutes on medium-high speed until light. Add the eggs and vanilla, beating well and scraping the bottom and sides of the bowl.
1 cup (227g) cold salted butter, 1 cup (200g) granulated sugar, 1/2 cup (100g) firmly packed light brown sugar, 2 large eggs, 2 teaspoons vanilla extract
Add both types of flour, cocoa powder, cornstarch, baking powder, baking soda, and salt. Mix on low speed just until combined. Stir in peanut butter chips.
1 1/2 cups (212g) all-purpose flour, 1 cup (120g) cake flour, 1/2 cup (43g) dark cocoa powder, 1 teaspoon cornstarch, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 2 1/2 cups Reese's peanut butter chips
Divide the dough into 8 evenly-sized large balls and space them evenly on the baking sheets so they have room to spread while baking.
Bake for 9-11 minutes until the cookies look set around the edges and dry on top. If they still look shiny, they probably need another minute, but don't overbake.
Let the cookies cool on the baking sheets for at least 15 minutes before transferring to a wire cooling rack to finish cooling.