Preheat oven to 350°F (175°C) and line a 9x13-inch pan with parchment paper.
In a large bowl, cream butter and sugar until light and fluffy.
Add eggs and vanilla extract, mixing until well combined.
In a separate bowl, whisk flour, baking powder, and salt.
Gradually mix dry ingredients into the wet mixture.
Press dough evenly into the prepared pan.
Bake for 15–18 minutes, or until edges are lightly golden.
Let cool completely in the pan.
For frosting, beat butter until smooth, then add powdered sugar, milk, and vanilla.
Divide frosting and tint with red and blue coloring as desired.
Spread frosting over cooled bars and decorate with sprinkles.
Slice into squares and serve.