NOTE: It is SUPER IMPORTANT that all the room temperature ingredients listed above are room temperature and measured by weight so that the ingredients mix and incorporate correctly. Heat oven to 335º F/168º C — 350º F/177º C. I tend to use lower setting to prevent my cakes from getting too dark on the outside before the inside is done baking.
Prepare three 6"x2" cake pans with cake goop or preferred pan spray. Fill your pans about 3/4 of the way full of batter. You might have a little batter leftover. You can make these into cupcakes!
Whisk together the flour, sugar, baking powder, baking soda and salt in the bowl of a stand mixer with the paddle attachment. Mix 10 seconds to combine.
Next, combine 1/2 cup of the milk and the oil together and set aside.
Combine the remaining milk, egg whites and vanilla together, whisk to break up the eggs and set aside.
Add your softened butter to the dry ingredients and mix on low until mixture resembles a coarse sand (about 30 seconds). Add in your milk/oil mixture and let mix until dry ingredients are moistened and then bump up to med (setting 4 on my kitchenaid) and let mix for 2 minutes to develop the cakes structure. If you don't let your cake mix on this step your cake could collapse.
Scrape your bowl and then reduce speed to low. Add in your egg white mixture in three batches, letting the batter mix for 15 seconds between additions.
Divide cake batter into four bowls. Add 1-2 tsp of food coloring to each bowl and stir until just combined.
Divide batter into three 6" cake pans prepared with cake goop or another preferred pan release. Bake 30-35 minutes until a toothpick inserted into the center comes out cleanly but the cake has not begun to shrink yet from the sides of the pan.
IMMEDIATELY TAP PAN FIRMLY on countertop once to release the steam from the cake. This stops the cake from shrinking.
Let cakes cool for 10 minutes inside the pan before flipping them out. Flip onto a cooling rack and let cool fully. I chill my cakes in the freezer for one hour before using or you can wrap in plastic wrap and chill in the fridge overnight.