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 12 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.