Cream together the unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. Beat in the egg and vanilla extract until fully combined.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt.

Gradually add the dry mixture to the wet ingredients, mixing on low speed until fully integrated. Use a spatula to ensure no dry spots remain.

Divide dough into two portions, roll into logs, wrap in plastic wrap and refrigerate for 1-2 hours.

Preheat oven to 350°F (175°C). Dust surface with cocoa powder, roll out dough, cut shapes using cookie cutters and place on parchment-lined baking sheets.

Bake for 10-12 minutes until edges are firm. Let cool on baking sheet for 5 minutes before transferring to wire rack.

Once completely cool, decorate with royal icing and sprinkles.