Cream butter and sugars in a medium bowl with an electric mixer until fluffy, about 3 minutes. Add eggs, vanilla, and cocoa and beat well.
Gradually add flour, salt, and baking powder until just combined. Cover with plastic wrap and chill for 1 hour.
Preheat the oven to 350 degrees F and lightly grease a baking sheet. Roll out half the dough to 1/4-inch thickness on a floured surface and use a 2-inch cookie cutter to cut out rounds. Ball up the scraps and repeat. Bake cookies until just set in the center and a little puffed, about 10 minutes. Move to a rack to cool. Continue with remaining dough.
Pour the sprinkles, sanding sugar, and/or crushed rock candy into shallow bowls. Put a heaping tablespoon of frosting on a cookie and top it with a second cookie, pressing down to form a smooth seal. Roll the frosted edge in a decoration of your choice. Makes 24.