Step 1: Preheat the oven to 325˚ F. Line two cupcake tins with paper liners.
Step 2: In a medium bowl, combine 3 cups of all-purpose flour, 1 tablespoon of baking powder, and ½ teaspoon of salt. Stir to blend.
Step 3: Cream together 1 cup of unsalted butter and 2 cups of sugar until light and fluffy, about 3 to 5 minutes.
Step 4: Blend in 4 large eggs one at a time, mixing well after each addition.
Step 5: Mix in 1 cup of buttermilk and 1 teaspoon of vanilla extract.
Step 6: Gradually incorporate the dry ingredients into the wet mixture in three additions.
Step 7: Divide the batter into four bowls and color each with food coloring to create an ombre effect.
Step 8: Fill each cupcake liner with the colored batter, starting with the darkest color.
Step 9: Bake for 20 to 24 minutes, or until a toothpick comes out clean.
Step 10: Cool in the pan for 5 to 10 minutes before transferring to a cooling rack.
Step 11: Prepare the frosting by whisking together 3 tablespoons of flour and ½ cup of whole milk until thickened, then cool completely.
Step 12: In a mixer, beat ½ cup of unsalted butter and ½ cup of granulated sugar until fluffy, then add the cooled flour mixture and 1 teaspoon of vanilla extract.