In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.

In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and creamy, about 2 minutes.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet mixture, mixing on low speed. Continue until the dough comes together and no streaks of flour remain.

Divide the dough into two portions. Roll each portion to ¼-inch thickness between two sheets of parchment paper. Stack the rolled dough (with parchment paper) and chill in the refrigerator for 1-2 hours.

Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper. Remove the dough from the refrigerator and cut into shapes using cookie cutters. Arrange cookies on prepared baking sheets, leaving space between each. Bake for 11-12 minutes, or until edges are set. Cool on the baking sheet for 5 minutes, then transfer to a wire rack.

Once cooled, decorate cookies with royal icing, glaze, or sprinkles as desired. Allow icing to set before serving or storing.