Whisk flour and cocoa together in a small bowl to aerate and combine; set aside.

In a large bowl with an electric mixer on medium-high speed, beat butter until creamy, about 4 minutes, scraping down the bowl as needed. Add confectioners’ sugar, beating until light and fluffy, about 4 minutes more, scraping down the bowl once or twice; beat in vanilla. Add about one-third of flour mixture and mix on low speed. Gradually add remaining flour mixture, mixing until well blended. This might take some time and will depend on the softness of your butter and the power of your mixer. Just keep mixing until well blended.

Roll each cookie into a small 1-inch (2 cm) ball. This will go most quickly if you use a tiny scoop. Place cookies spaced evenly apart on sheet pans and press a few morsels onto the top of each cookie, then flatten each cookie using your fingers or palm to 1/2-inch (12 mm) thickness.

Bake for about 15 to 20 minutes or until dry to the touch. They will not brown but you should be able to lift them gently from the parchment paper with a spatula without them breaking. Cool pans on racks. Store cookies in airtight containers at room temperature for up to 1 week.