Preheat oven to 350 degrees. Then spray 3 8-inch cake pans with non-stick spray and line the bottoms with parchment paper.

In a large bowl, whisk together the gluten free flour, cornstarch, xanthan gum, baking powder, baking soda, salt, and 1 1/2 cups (340 g) granulated sugar. Set aside.

In a medium bowl, whisk together the egg yolks, buttermilk, melted butter, oil, vanilla extract, and food coloring (if using). Set aside.

Add the egg whites and cream of tartar to a separate, clean medium bowl. Beat with an electric mixer on medium-low speed until the egg whites become thick and foamy. Slowly add in the remaining 1/4 cup (60 g) granulated sugar until the meringue is shiny and holds a stiff peak. Then set the meringue aside.

Pour the buttermilk mixture into the dry ingredients and mix until incorporated. Add half of the meringue to the batter and gently fold in with a spoon. When it's mostly folded in, add the remaining meringue. Gently fold in until no white streaks remain. Make sure you're folding and not stirring/mixing so you don't deflate the meringue. It may take a minute or two to fully incorporate the meringue.

Transfer the batter to the prepared pans, making sure it's distributed evenly between the three and spread the batter evenly in the pans. Bake at 350 degrees for 20-22 minutes.

Cool the cakes in the pan for 25 minutes then remove them from the pans and cool completely on wire racks. Once the cakes are cooled, frost them with chocolate buttercream frosting or vanilla buttercream frosting.