Preheat oven to 400 degrees F and line the wells of a 48-cup mini cupcake pan with paper mini cupcake liners.
Whisk together flour, baking powder, and salt in a medium bowl until well combined .
In the bowl of a stand mixer, cream butter and sugar together on medium speed until fluffy. Add eggs and vanilla and continue mixing until well combined.
Add half of dry ingredients to the wet ingredients and mix on low speed until the flour just starts to incorporate. Add half of the milk mixture followed by the rest of the dry ingredients and the rest of the milk. Do not overtax batter.
Drop batter by tablespoon into the prepared muffin tin, filling each well evenly. Bake cupcakes in the oven for 8 minutes, or until a toothpick inserted into a center cupcake comes out clean. Remove cupcakes from oven and transfers for a baking rack. Cool completely.
Just before serving (or no more than 8 hours before serving), combine the Pop Rocks Candies and sugar sprinkles in a small bowl. Use a cupcake corer or a small spoon to carefully remove the center of each cupcake. Fill the hole with a spoonful of the candy mixture. Replace the top of the cupcake (you may need to cut the cake plug in half).
Frost the cupcakes as follows, using the icing recipe below:
- Frost 12 cupcakes in blue frosting and add white stars at random using a small white star tip.
- Frost 20 cupcakes in red frosting using a blunt tip.
- Frost the remaining cupcakes with white frosting using a blunt tip.
- Arrange cupcakes in a flag shape on a large platter or tray.