Begin by preheating the oven to 300 degrees F (150 degrees C) and greasing and flouring a 10-inch springform pan, making sure to use gluten-free flour. Next, combine the gluten-free baking flour, cornstarch, cocoa powder, and baking powder in a bowl, sifting the mixture if necessary to remove any clumps.
In a food processor, add the butter, avocado, applesauce, sugar, vinegar, buttermilk, and vanilla extract, and pulse several times until well combined. Mix in the egg yolks and process again, then transfer the mixture to a large mixing bowl.
Using an electric mixer, beat the egg whites with cream of tartar until stiff peaks form, then gently fold them into the avocado mixture. Next, fold in the flour mixture, being careful not to deflate the batter. Add red food coloring and mix gently.
Pour the batter into the prepared springform pan and bake in the preheated oven for about 1 1/2 hours, or until a toothpick inserted into the center of the cake comes out clean. After baking, allow the cake to cool. Keep in mind that since the cake is gluten-free, it may sink slightly while cooling. Once cool, remove the cake from the springform pan for decorating.
