Instructions:
- Preheat oven to 350°F (175°C). Grease and flour two 8-inch pans, line bottoms with parchment circles.
- Sift and whisk together cake flour, sugar, baking powder, and salt in a large bowl.
- Add softened butter to the dry ingredients. Mix on low speed until the mixture resembles coarse sand.
- In a separate bowl, whisk together milk, buttermilk, and vanilla extract.
- With the mixer on low, gradually add half the wet mixture to the flour/butter mixture. Mix until just combined.
- Scrape down the bowl. Add the remaining wet mixture and mix only until the batter is smooth and no streaks remain. Do not overmix.
- In a clean, grease-free bowl, beat the egg whites until stiff, glossy peaks form.
- Gently fold the egg whites into the cake batter in two additions using a spatula, preserving as much air as possible.
- Divide batter evenly between prepared pans. Bake for 30–35 minutes, or until a toothpick inserted comes out clean. Cool in pans for 10 minutes, then invert onto a wire rack to cool completely.
- For the frosting: Beat the very soft butter until light and fluffy. Gradually add the sifted powdered sugar, one cup at a time.
- Add vanilla and salt. Beat until combined, then add heavy cream one tablespoon at a time until the desired spreading consistency is reached.
- Assembly: Ensure cakes are completely cool. Place the first layer on a serving plate, spread frosting, and top with the second layer. Apply a thin crumb coat and chill for 15 minutes before applying the final, smooth layer of frosting.