Preheat oven to 350°F.
In a medium bowl, sift the flour, sugar, baking powder, and salt together. Set aside.
4 cups cake flour, 2 cups granulated sugar, 1 tbsp baking powder, 1 tsp kosher salt
In a large bowl or stand mixer fitted with the paddle attachment, beat the butter until it’s smooth. Add in the vanilla and eggs.
1 cup unsalted butter, 1 teaspoon pure vanilla extract, 8 large egg whites
Switch to the whisk attachment and continue beating the mixture on medium speed for about 30-60 seconds, incorporating a small amount of air into the egg whites. (Don’t overmix until they are peaks; just add enough air for it to begin to slightly foam.)
Add the flour mixture and milk, continuously mixing on slow to medium speed. Be sure to scrape the sides of the bowl to incorporate all of the dry ingredients.
1½ cups milk
Once the batter is smooth and creamy, evenly divide the batter into three lightly buttered and floured 8-inch round cake pans.
Bake for 25-28 minutes, using a toothpick to check for doneness. Let the cakes cool in their pans for about 5 minutes, then remove from the pans to let them cool completely on a wire rack.
Before assembling, cut the tops off of each layer to have even and flat layers.
To assemble the cake, pipe an outline of frosting around the top of the cake, and then fill in the center. Lightly add the next cake layer, and gently press it into the bottom layer. Repeat with the next cake until all three layers are stacked with a layer of frosting in between each layer.
2½ cups buttercream frosting
Lightly frost the entire outer cake with a crumb coat. This will make the cake look cleaner and well covered.
To make the rosettes, begin with the bottom row. Fill a large decorating bag fitted with a large star tip (I used Wilton 2D) with buttercream frosting.
Starting at the bottom of the cake, hold the bag straight up with the tip just next to the cake, squeeze the bag and complete a full loop. Once you get to the end of the rosette shape, release pressure to the bag so you can pull the bag away.
Repeat rosettes until you have gone all the way around the cake. Then begin the next row. Continue until the entire cake is covered in rosettes. Any small uncovered areas, gently fill in with small rosettes.