01 - Preheat oven to 350°F. Line a 12-cup muffin tin with decorative cupcake liners.
02 - In a bowl, whisk together all-purpose flour, baking powder, and salt until well blended.
03 - In a separate mixing bowl, beat softened butter and granulated sugar with an electric mixer on medium speed until pale and fluffy. Add eggs one at a time, mixing thoroughly after each addition, then blend in vanilla extract.
04 - Add flour mixture to the creamed butter in thirds, alternating with whole milk. Begin and finish with the flour, mixing gently after each addition just until incorporated.
05 - Distribute batter evenly among the lined muffin cups. Bake for 18 to 20 minutes or until a toothpick inserted in the center emerges clean. Allow cupcakes to cool completely on a wire rack.
06 - In a clean bowl, beat unsalted butter until creamy. Gradually add sifted powdered sugar, two tablespoons milk, and vanilla extract. Increase speed and beat until frosting is fluffy and smooth.
07 - Divide buttercream evenly among three bowls. Tint one portion with red gel coloring and another with blue, leaving one bowl white. Fill a piping bag fitted with a large star tip, alternating spoonfuls of each color to achieve a swirl pattern.
08 - Pipe tall swirls of tri-color buttercream onto each cooled cupcake. Adorn with red, white, and blue sprinkles, and star-shaped candies if desired. Add sparklers or themed toppers just before serving, removing all decorative items prior to eating.