Cream together butter and sugar in a bowl until light and fluffy, beat in egg and vanilla extract.

In a separate bowl, whisk together flour, baking soda, and baking powder. Incorporate dry ingredients into wet mixture, chill dough for at least 1 hour.

Preheat oven to 375°F (190°C). Roll out dough to ¼-inch thickness, cut circles and create cavity shapes with a sharp knife.

Bake cookies in the preheated oven for 10-12 minutes or until lightly golden, cool on a wire rack.

Whisk powdered sugar with milk and vanilla until smooth. Fill cavity of each cookie with icing, sprinkle rock candy crystals immediately.

Allow cookies to dry for at least 2 hours at room temperature before storing.