**Prepare the Dough:**
In a large mixing bowl, cream the butter and sugar together with an electric mixer until light and fluffy, about 2-3 minutes. Beat in the egg and vanilla extract until combined.
**Mix Dry Ingredients:**
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a dough forms.
**Chill the Dough:**
Divide the dough into two discs, wrap each in plastic wrap, and refrigerate for at least 1 hour or until firm.
**Roll and Cut:**
Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper. Roll out the dough on a lightly floured surface to about 1/4-inch thickness. Use cookie cutters to cut out desired shapes and place them on the prepared baking sheets, spacing them about 1 inch apart.
**Bake:**
Bake in the preheated oven for 8-10 minutes, or until the edges are set. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
**Decorate:**
Once cooled, decorate the cookies with royal icing, melted chocolate, or sprinkles as desired.