Preheat the oven to 350 degrees. Line 14 to 15 muffin cups with paper liners and set pans aside.

Whisk together the flour, baking powder, baking soda, and salt in a small bowl. (These are your dry ingredients.) Set aside.

Combine the butter and sugar in a large bowl and beat with an electric mixer on medium until fluffy and pale yellow in color, about 5 minutes. (Scrape down the bowl with a rubber spatula as necessary.)

Beat in the egg whites and vanilla to combine.

Turn mixer to low and beat in half of the dry ingredients followed by half of the buttermilk. Scrape down the bowl.

Mix in the remaining dry ingredients, then the remaining buttermilk, mixing just until incorporated. You don’t want to over-mix your batter or your cupcakes could turn out tough or heavy.

Divide batter between the prepared muffin cups, filling about 2/3 full. Bake 14 to 20 minutes, until a cake tester inserted into the center of the cupcakes comes out clean.

Cool in the pans for 5 minutes, then remove to a wire rack to cool completely before frosting.

Place the butter in a medium mixing bowl and beat with an electric mixer until very smooth. Add 4 tablespoons of milk, the vanilla, and the salt and beat to combine.

With mixer on low, gradually beat in 3 cups of the powdered sugar. Turn mixer to medium and beat until frosting is smooth. Add add additional powdered sugar if frosting is too thin; if it’s too thick add additional milk.

Use a small paintbrush to brush a stripe of red coloring on one side of a large piping bag that’s been fitted with a large round tip. Use a second paintbrush (or wash and dry the first one well) to brush a stripe of blue on the opposite side. The stripes should be about 1/4-inch thick but a little thicker or thinner doesn’t matter.

Pipe the frosting onto the cooled cupcakes, then top with sprinkles (if desired). Serve.

Uneaten cupcakes can be stored at room temperature, covered, for up to 3 days. You can also freeze them, tightly wrapped, for up to 2 months.