Prepare cake pans by lining the bottoms with parchment paper. You can use either three 8" cake pans, or three 9" cake pans. Preheat the oven to 350 degrees.

In a standing mixer, whisk butter and sugar until light and airy. Approximately 3-5 minutes, stopping at 2 minutes to scrap down the sides.

In a separate bowl, whisk together the egg, egg whites, sour cream, milk and vanilla.

In a third bowl, whisk together the GF flour, baking powder, baking soda, potato starch, guar gum and salt.

Set the mixer to medium/low speed, and alternate adding the wet and dry ingredients to the butter and sugar mixture, starting and ending with the wet ingredients.

Once combined, turn off the mixer. The batter will be smooth and thick.

Divide the batter evenly between the cake pans and use the angled icing spatula to smooth the batter into a flat layer.

Pop the cake pans into the oven and prep your frosting while you wait for these beauties to bake! Bake for 15 minutes, rotate, then bake another 15 minutes. When cakes are golden brown, and you can insert a toothpick into the middle with only crumbs sticking to it – they are ready!

Let cakes cool for 15 minutes before removing from the cake pans. Place the cakes on a cooling rack. They must be completely cool before adding frosting to the cakes.