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

In a medium bowl, whisk together the flour, salt, baking powder, and baking soda. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium-high speed until light and fluffy, about 3 minutes.

Add the eggs one at a time, beating well after each addition. Scrape down the bowl as needed. Add the sour cream and vanilla and beat until combined. Gradually add the dry ingredients and 2/3 cup buttermilk in three additions, starting and ending with the dry ingredients. Mix just until combined.

Scoop out 1 cup of the batter and stir in the cocoa powder and the remaining 2 tablespoons buttermilk. Divide this chocolate batter in half and tint one half with red food coloring and the other with blue. Place a teaspoon of red batter and a teaspoon of blue batter into each cupcake liner. Top with 1 tablespoon of the white batter. Repeat layering with the red, blue, and white batters until the liners are about 2/3 full (you should use almost all the batter).

Gently tap the pan on the counter to level the batter. Use a toothpick to swirl the colors together for a marbled effect. Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until soft, about 30 seconds. Add the sifted powdered sugar, attach the splash guard, and mix on low speed until the sugar is just incorporated, about 3 minutes. The mixture will look powdery at first. Scrape down the sides and bottom of the bowl, then mix again to incorporate any loose sugar.

Turn the mixer to high, immediately add the vanilla and hot water, and mix until smooth, about 20 seconds. If desired, transfer the buttercream to a piping bag and decorate the cooled cupcakes as you like.