Preheat the oven to 350°F, and line a cupcake pan with 12 paper liners.
In a large bowl, use an electric mixer to combine the butter and sugar until light and fluffy. Add the eggs, one at at time, beating well after each addition. Beat in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder and salt. On low speed, alternately add the flour mixture and milk, in three additions, beginning and ending with the flour. Scrape down the sides of the bowl.
Split the batter evenly into three small bowls. Add red, white and blue gel paste until you achieve the colors that are desired. (The white won't turn completely white, but a squirt or two will lighten up the batter a little bit.) Spoon the batter into the muffin cups- a spoonful of red, white and blue in each cup. Use a toothpick to gently swirl the batter in each cup.
Bake 16 to 20 minutes, or until a toothpick inserted into the center comes out clean (the center should puff up slightly and not feel gooey-- and it should feel springy to touch when done). Cool completely before icing.
PREPARE THE ICING:
In a large bowl, with a hand or stand mixer, cream together sugar and butter on low speed. Mix until well blended. Increase speed to medium and beat for another 3 minutes. Add vanilla and 1 tablespoon of cream and continue to beat on medium speed for 1 minute more. Add more cream if needed for desired spreading/piping consistency.
FROST THE CUPCAKES:
Divide the frosting into 3 bowls and color red/blue. Put red/white/blue icing in the SAME piping bag, and you'll get a nice swirl effect with all three colors. The Wilton 1M tip is a good one for creating the look in the photos.
Notes
*Use clear vanilla extract to get a whiter cupcake.
*Gel paste can be found in baking supply stores. You can also use the Wilton brand that is found in craft stores like Michaels.