Preheat the oven to 350 degrees, grease and flour three 8×2 inch round pans.
In a medium bowl, whisk the gluten free flour, baking powder, baking soda and salt for 30 seconds. Set aside
In another bowl, add the buttermilk, vegetable oil and vanilla extract. Set aside..
In the bowl of your mixer, add the butter and mix at medium speed until smooth. Gradually add the sugar and continue to mix on medium speed for 3 to 5 minutes until lightened in color and fluffy.
Add the eggs one at a time mixing until the yellow of the yolk is blended.
With the mixer on low speed, alternately add the flour mixture and buttermilk mixture, beginning and ending with the dry ingredients (3 additions of dry ingredients and 2 of liquid). After the last addition, mix just until combined. Be careful not to over-mix.
Divide the cake batter between the three prepared cake pans.
Bake at 350 degrees for 25-30 minutes or until a toothpick inserted in the center comes out clean or with just few crumbs attached. Let cool 10 minutes and turn out.
For the Vanilla Buttercream
Cream the (slightly softened) butter until smooth. Blend in the vanilla.
Add half of the powdered sugar and most of the milk. Beat at medium speed until the powdered sugar is incorporated.
Add remaining powdered sugar and milk and mix at medium speed another 3 to 4 minutes scraping the sides of the bowl occasionally. I slow down the mixer to very slow. (#2 on the Kitchenaid) for 1 to 2 minutes. This will help eliminate air pockets in the buttercream. The texture will become very smooth.
This recipe can be doubled or halved.
Yields approximately 4 ½ cups of frosting. Enough to frost a two layer 8 or 9 inch cake.
Can be frozen in air tight container for at least three months . Thaw on countertop.
If your buttercream is too thin, add more sugar. If the consistency is too thick, add a bit more milk.