Set the oven rack at the middle level and Preheat the oven to 180℃
In a bowl, sift and combine the flours, corn starch, xanthan gum, baking powder, baking soda, and salt.
Using a stand-alone mixer with the paddle attachment or hand-held mixer, beat the sugar and butter at medium-high speed until light and fluffy. Then, add the egg whites and beat until creamy.
Next, add the apple cider vinegar, sour cream, and vanilla extract. Beat at low-medium speed until combined.
Then, at low speed, add one-third of the dry ingredients, followed by one-third of the milk. Then add half of the remaining dry ingredients followed by half the milk. Next, add the remaining dry ingredients and milk, beating until combined after each addition. Don’t overbeat the batter.
Grease three 8-inch cake tins and align them with baking paper. Fill each cake tin ⅓ full.
Bake the cake layers for 25-30 minutes or until a toothpick inserted at the center comes out clean.
Allow the cake layers to cool in the cake tin for about 5 minutes before transferring it to a cooling rack. Cool the cake layers to room temperature before decorating.