01 - Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl. Set aside.
02 - Using an electric mixer, beat softened butter and granulated sugar until light and fluffy, about 2 to 3 minutes.
03 - Beat in the egg and vanilla extract until fully combined.
04 - Gradually add dry ingredients to the wet mixture, mixing on low speed until a soft dough forms.
05 - Divide dough in half, shape each into a disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour or overnight.
06 - Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper. Combine granulated sugar and colored sanding sugar in a shallow bowl.
07 - Scoop tablespoon-sized dough portions, roll into balls, then roll in the sugar mixture to coat evenly.
08 - Place dough balls 2 inches apart on the prepared sheets and gently flatten each with the palm of your hand.
09 - Bake for 9 to 11 minutes until edges are firm and centers remain slightly soft.
10 - Allow cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.