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

Mix Dry Ingredients:
In a medium bowl, whisk together flour, baking powder, and salt.

Cream Butter & Sugar:
In a large bowl, cream butter and sugar using an electric mixer until light and fluffy, about 3–4 minutes.

Add Egg Whites & Vanilla:
Beat in egg whites one at a time, mixing well after each. Stir in vanilla extract.

Combine Wet & Dry:
Add the flour mixture and milk alternately in three additions, starting and ending with the dry ingredients. Mix until just combined—do not overmix.

Bake:
Divide batter evenly among the prepared pans. Bake for 22–28 minutes or until a toothpick inserted in the center comes out clean.

Cool Cakes:
Let cakes cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Make Frosting:
Beat softened butter until creamy. Gradually add powdered sugar. Add vanilla, heavy cream, and salt. Beat until fluffy, adding more cream if needed to reach desired consistency.

Assemble & Decorate:
Place one cake layer on a serving plate. Spread frosting evenly on top. Repeat with remaining layers. Frost the top and sides of the cake. Decorate with piped swirls or sprinkles if desired.