Preheat oven to 350°F (177°C). Grease two 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper.
Whisk the flour, baking soda, cocoa powder, and salt together in a large bowl. Set aside.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until combined, about 1 minute. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed.
Add the oil, egg yolks, vanilla extract, and vinegar and beat on high for 2 minutes. Set the egg whites aside. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed
With the mixer on low speed, add the dry ingredients in 2-3 additions alternating with the buttermilk. Beat in your desired amount of food coloring just until combined.
Vigorously whisk or beat the 4 egg whites on high speed until fluffy peaks form, about 3 minutes. Gently fold into cake batter. The batter will be silky and slightly thick.
Divide batter between cake pans. Bake for 30-32 minutes or until the tops of the cakes spring back when gently touched and a toothpick inserted in the center comes out clean.
Let cakes cool completely in the pans set on a wire rack before frosting and assembling
Frost the cake [33-36].
Refrigerate cake for at least 30-60 minutes before slicing