In a medium bowl, whisk together the cocoa powder, salt, baking powder, and 2½ cups flour.
¾ cup unsweetened cocoa powder, 1 ½ teaspoons kosher salt, 1 teaspoon baking powder, 2 ½ cups all-purpose flour
In the bowl of a stand mixer, beat the butter, brown sugar, and granulated sugar on medium-high speed until light and fluffy, about 4 minutes. 1 cup unsalted butter, 1 cup light brown sugar, ¾ cup granulated sugar
Reduce mixer speed to medium and add eggs and vanilla extract; beat until very fluffy, about 30 seconds.
2 large eggs, 2 teaspoons vanilla extract
Reduce speed to low and gradually add dry ingredients, beating just until combined (use a to incorporate any dry ingredients at the bottom of the bowl).
Divide dough in half and wrap each in plastic wrap, pressing down to form into ½”-thick disks. Chill until firm, at least 2 hours.
Preheat the oven to 350°.
Let 1 disk of dough sit at room temperature until softened slightly, about 5 minutes.
Roll out on a lightly floured piece of parchment paper into a large ⅛"-thick rectangle (about 16x12"), dusting lightly with more flour as needed. Use a 3 inch round cookie cutter to cut out as many rounds as you can fit, then use a thin metal spatula to transfer rectangles to a parchment-lined baking sheet, spacing 1" apart. (If at any point the dough becomes too soft and sticky, slide dough with parchment onto a baking sheet and chill until firm.) Chill dough on baking sheet while you repeat the process with the second disk of dough.
You should end up with approximately 30 cookies total. Save scraps for another use or discard.
Bake, rotating baking sheets front to back and top to bottom halfway through, until edges are firm, 12–15 minutes.
Let cool on baking sheets. Turn half of cookies over so the flat side is facing up.