Preheat oven to 350°F (175°C). Grease and flour three 8-inch round cake pans or line with parchment paper.

In a large bowl, whisk together flour, baking powder, and salt. Set aside.

In a separate bowl, cream together softened butter and granulated sugar until light and fluffy.

Beat in eggs one at a time, then stir in vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with dry ingredients. Mix until just combined.

Divide the batter into three equal portions. Add red gel food coloring to one portion, blue gel food coloring to another, and leave the third portion white.

Pour each colored batter into its own prepared cake pan. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let cool in pans for 10 minutes before transferring to a wire rack to cool completely.

In a large bowl, beat softened butter until creamy.

Gradually add powdered sugar, one cup at a time, beating well after each addition. Add vanilla extract and heavy cream, continuing to beat until light and fluffy.

Once cakes are completely cool, level the tops with a serrated knife if needed.

Place the blue layer on a serving plate or cake stand. Spread a layer of buttercream on top. Add the white layer, spread with buttercream, then top with the red layer.

Frost the entire cake with the remaining buttercream. Decorate with sprinkles or fresh berries if desired.