Preheat the oven. Heat oven to 375°F (190°C). Line two baking sheets with parchment paper.
Mix dry ingredients. In a medium bowl, whisk together flour, black cocoa powder, baking soda, baking powder, and salt. Set aside.
Cream butter and sugar. In a large mixing bowl, beat butter and sugar together with a hand or stand mixer until light and fluffy, about 2–3 minutes.
Add egg and vanilla. Beat in the egg and vanilla extract until smooth. Scrape down the sides of the bowl.
Combine wet and dry ingredients. Add the dry mixture to the wet ingredients in two parts, mixing just until combined. Dough should be thick and dark but not sticky.
Add sprinkles. Fold in Halloween sprinkles for pops of color throughout the dough.
Scoop and roll. Scoop dough using a 1-tablespoon cookie scoop. Roll into balls, then coat in granulated sugar if desired. Place 2 inches apart on prepared baking sheets.
Bake. Bake for 9–11 minutes, until the edges are set and the centers look slightly soft. Do not overbake — they’ll continue setting as they cool.
Decorate. Immediately press candy googly eyes into warm cookies before they firm up.
Cool. Let cookies cool on the baking sheet for 2–3 minutes, then transfer to a wire rack to cool completely.
