Step 1

Soften the butter. Place 6 tablespoons unsalted butter in the bowl of a stand mixer. (Alternatively, place in a large bowl if using an electric hand mixer.) Place 6 large egg whites in a medium bowl. Let both sit out at room temperature until the butter is softened.

Step 2

Heat the oven and prepare the pans. Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat 2 (8- or 9-inch) round cake pans with cooking spray, then line the bottoms with parchment paper rounds.

Step 3

Mix the dry ingredients. Place 2 2/3 cups all-purpose flour, 1 tablespoon baking powder, and 1/2 teaspoon kosher salt in a medium bowl and whisk to combine.

Step 4

Mix the wet ingredients. Add 2 cups granulated sugar, 2/3 cup canola oil, and 2 teaspoons vanilla extract to the bowl of butter and beat with the paddle attachment on medium-high speed until combined, 1 to 2 minutes. The mixture will look somewhat grainy at this point.

Step 5

Add the dry ingredients. Add half of the flour mixture and 1/2 cup of the whole milk. Mix on medium speed until combined, 1 to 2 minutes. Add the remaining flour mixture and remaining 1/2 cup whole milk, and mix on medium speed until completely smooth, 2 to 3 minutes. If using a stand mixer, transfer the batter to a large bowl and wash and dry the stand mixer bowl.

Step 6

Whip the egg whites. Place 6 large egg whites in the clean bowl of the stand mixer. (Alternatively, use an electric hand mixer and a second large bowl.) Whip with the whisk attachment on medium-high speed until foamy, 1 to 2 minutes. Increase the speed to high and whip until stiff peaks form, 2 to 3 minutes.

Step 7

Fold the egg whites in. Add the egg whites to the bowl of cake batter and gently fold with a rubber spatula by hand until fully incorporated with no visible streaks of egg whites left.

Step 8

Bake the cakes. Evenly divide the batter between the prepared baking pans. Bake until the cakes are lightly browned and a toothpick inserted into the center comes out clean, 30 to 35 minutes for 9-inch pans, or 35 to 40 minutes for 8-inch pans. Place the pans on a wire rack and let cool completely.

Step 9

Assemble the cake. Run a thin knife around the outside of the cakes. Invert and remove them from the pans. Discard the parchment. If the tops of the cakes have domed, use a serrated knife to trim them.

Step 10

Place one cake layer, cut-side down, onto a serving platter or cake stand. Dollop 1 cup of the frosting onto the cake and smooth into a thin, even layer with an offset spatula, going all the way to the edges. Top with the remaining cake layer, cut-side down. Frost the top and sides of the cake with the remaining frosting.