In a medium mixing bowl, whisk the flour, cocoa powder, salt and baking powder together. Then set aside the flour mixture.
1 ½ cups (188 g) all-purpose flour, spooned and leveled, ½ cup (40 g) Dutch-process cocoa powder, ½ teaspoon baking powder, ½ teaspoon sea salt
In a large bowl, cream the butter, brown sugar, and granulated white sugar together with an electric mixer on high speed. (You can also use a stand mixer with a paddle attachment.)
¾ cup (168 g) unsalted butter, softened, ¾ cup (165 g) light brown sugar, packed, ¼ cup (50 g) granulated white sugar
Add in the egg yolks and vanilla and mix on medium speed until light and fluffy, about 1-2. minutes.
2 egg yolks, at room temperature, 1 ½ teaspoon vanilla extract
Add the dry ingredients to the wet ingredients and mix on low speed just until combined.
Scoop the dough in 32 portions with a tablespoon-size cookie scoop and roll into balls. Directly after rolling into a ball, press each with a ¼ teaspoon to make an indent. Chill the dough for one hour.
Preheat the oven to 350 degrees 15 minutes before you're ready to bake the cookies.
Arrange 8 cookies per large, parchment paper lined baking sheet.
Bake the cookies for 9-11 minutes. (I think 10 is perfect.)When the cookies are straight out of the oven lightly press down on the centers again with a ¼ tsp. (They may have lost their indent a little as they baked.) Scoot a circular cookie cutter/biscuit cutter around each cookie while they are still hot to give them a perfect circular shape, and to get them back to their original smaller size. Let the cookies cool on the baking sheet for five minutes, then transfer them to a wire rack to completely cool.