Preheat oven to 400°F. Line a baking sheet with parchment paper.
Beat butter, brown sugar, and granulated sugar in a stand mixer fitted with the paddle attachment or using a handheld mixer for 2-3 minutes until creamy and light. Add egg yolk and vanilla, then beat again, stopping to scrape the bottom and sides of the bowl.
4 Tablespoons cold salted butter, 1/4 cup packed brown sugar, 3 Tablespoons granulated sugar, 1 large egg yolk, 1/2 teaspoon vanilla extract
Add flour, cocoa powder, baking powder, baking soda, and salt. Mix again just until combined. Stir in milk chocolate, semisweet, and white chocolate chips.
1/2 cup all-purpose flour, 2 Tablespoons unsweetened cocoa powder, 1/4 teaspoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon salt, 1/2 cup chocolate chips
Divide the dough evenly into 4 large or 6 regular sized cookies, spacing them a couple inches apart on the baking sheet so they have room to spread. Bake for 8-9 minutes until set around the edges and no longer shiny on top. You may want to press a few additional chips into the top of each cookie while they are still hot out of the oven for decoration.
Cool for 5-10 minutes on the baking sheet before transferring to a wire rack to cool completely. Enjoy!