Preheat oven to 300 degrees F (150 degrees C). Grease and flour a 10-inch springform pan, using gluten-free flour. In a bowl, sift together the gluten-free baking flour, cornstarch, cocoa powder, and baking powder. If mixture seems clumpy, sift again.
Place the butter, avocado, applesauce, sugar, vinegar, buttermilk, and vanilla extract into the work bowl of a food processor, and pulse several times; process until well blended. Mix in the egg yolks, and process again. Scrape the mixture into a large bowl.
In a separate mixing bowl, beat the egg whites with cream of tartar using an electric mixer until the whites form stiff peaks. Gently fold the egg whites into the avocado mixture, then fold in the flour mixture, being careful to retain as much fluffiness in the batter as you can. Gently mix in red food coloring. Pour the batter into the prepared springform pan.
Bake in the preheated oven until the center is set and a toothpick inserted into the cake comes out clean, about 1 1/2 hours. Allow to cool. Since the cake is gluten-free, it may sink a little while cooling. When cool, remove from springform pan for decorating.