Preheat your oven to 350°F (175°C). Grease and flour the 9-inch round cake pans and line the bottoms with parchment paper.

In a large mixing bowl, cream the softened butter and granulated sugar together using a hand or stand mixer on medium speed until light and fluffy, about 5 minutes.

Gradually add the egg whites and vanilla extract to the mixture, mixing until fully incorporated.

In a separate bowl, sift together the all-purpose flour, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Begin and end with the dry ingredients, mixing just until combined after each addition.

Divide the batter evenly between the prepared cake pans. Bake in the preheated oven for about 30 minutes or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then remove from pans and transfer to a cooling rack to cool completely.

While the cakes are cooling, prepare the vanilla buttercream. In a large bowl, beat the softened butter on medium speed until creamy.

Gradually add powdered sugar, heavy cream, vanilla extract, and a pinch of salt to the butter. Beat on medium-high speed until fluffy and well combined, about 3-4 minutes. Adjust consistency with more cream or powdered sugar as needed.

Once the cakes are completely cooled, place one layer on a serving plate. Spread a generous layer of vanilla buttercream on top, then place the second layer on top.

Frost the top and sides of the cake with the remaining vanilla buttercream, smoothing it out using a spatula. Optional: Decorate with sprinkles or edible flowers.