Preheat oven to 350°F. Line a 12-cup muffin tin with liners and set aside.

In a medium bowl, whisk to combine flour, salt, baking powder and baking soda. Set aside.

Use a stand mixer fitted with a paddle attachment, or a handheld mixer, to cream butter and sugar on medium-high speed until light and fluffy, about 3 minutes.

Add eggs one at a time, beating until well mixed. Scrape down bowl with a rubber spatula as needed. Add sour cream and vanilla and beat until combined. Alternate adding dry ingredients and 2/3 cup buttermilk in 3 batches, starting and ending with dry ingredients. Mix until just combined.

Scoop out 1 cup of batter and combine it with cocoa powder and remaining 2 tablespoons buttermilk. Divide that mixture in half and color with red and blue food coloring. Fill the cupcake liners with a teaspoon of red and blue batters. Top with 1 tablespoon of white batter. Repeat with red, blue and white batters until liners are 2/3 full (this should use almost all of the batter).

Tap the batter down to even out the surface. Use a toothpick to gently marble the cupcakes. Bake 18-22 minutes, or until a toothpick inserted into the center comes out clean. Let cupcakes cool 5 minutes in the pan, then transfer to a cooling rack to cool completely.

Place butter in the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed to soften, about 30 seconds. Add all of the sifted powdered sugar, attach splash guard, and mix on low speed until it just comes together, about 3 minutes. Mixture will look very powdery at first. Scrape down sides and bottom of bowl, and mix again to incorporate any loose powdered sugar.

Turn mixer on high, immediately add vanilla extract and hot water, and mix until just combined, about 20 seconds. If desired, transfer to a piping bag and decorate cupcakes as desired.