Preheat your oven to 350 degrees F. Place 18 cupcake liners in muffin tin cups. Set aside.
In a medium mixing bowl, stir together the flour, baking powder, baking soda, and salt. Set aside.
Add the butter and sugar to a large mixing bowl (or stand mixer bowl) and beat until smooth and fluffy, 1-2 minutes.
Add in the vanilla and eggs. Mix until evenly combined.
Add half the flour mixture to the butter mixture, and beat together. Pour in the milk and mix until combined. Add the remaining flour mixture and beat until smooth.
Pour in the sprinkles and mix by hand with a wooden spoon or sturdy rubber spatula.
Portion the batter into the prepared muffin tin cups, filling each cup 2/3 of the way full. (I like to use a small cookie scoop for this.) Tap the muffin tin on the counter to help the batter form into the cups.
Bake for 13 to 17 minutes or until a toothpick inserted in the center comes out clean.
Carefully remove the cupcakes from the oven. Transfer to a wire rack and allow to cool completely.
Once the cupcakes have cooled, Place the butter in a large mixing bowl. Beat the butter until fluffy, about 1 minute.
Add 1 cup of powdered sugar to the butter. Mix until just combined. Add another cup of powdered sugar to the butter, and mix again. Repeat with the remaining powdered sugar.
Add the salt, vanilla, and milk to the butter mixture, and mix until smooth and evenly combined.
Place a piping tip and coupler on an icing bag. Fill the bag 3/4 full with frosting. Pipe the frosting onto the cupcakes.
Top the cupcakes with more red, white & blue sprinkles. (I like to gently dip the tops of the cupcakes in the sprinkles to help them really stick.) Refrigerate until ready to serve.
If your frosting is a little on the soft side (which is common during the summer months) you can refrigerate it for 30 minutes before icing the cupcakes.