Preheat oven to 375°F.

Line a 6-cup muffin/cupcake pan with paper or parchment liners. Measure out all ingredients and bring butter, milk, and egg white to room temp.

Add softened butter to a medium bowl and whip until creamy. Add sugar and mash into the butter. Cream together butter and sugar until light and fluffy, 1-2 minutes.

Add egg white and vanilla. Beat on medium until smooth.

Add half the flour (¼ cup) to the mixing bowl along with baking powder and salt. Mix on low until incorporated.

Mix in milk on low. Add remaining flour and mix on medium until well-mixed and smooth.

Divide batter between prepared cupcake cups, filling just under 2/3 full. Bake for 17-19 minutes, until a toothpick inserted into the center comes out clean or with just a few dry crumbs.

Once cool enough to handle, transfer cupcakes to a wire cooling rack to cool completely before frosting.

In a large mixing bowl, beat the butter until smooth and creamy. Add powdered sugar and mix on low speed, then increase to medium speed until fully combined.

Add heavy cream, vanilla extract, and salt; continue mixing on medium speed until well combined.

Divide and dye frosting to desired colors with gel food coloring. Transfer to a piping bag for decorating. Frost cupcakes once cooled completely.