Grease and flour three 8x2 inch round pans and preheat oven at 350 degrees.Whisk gluten-free flour, baking powder, baking soda, and salt for 30 seconds in a medium bowl. Set aside.
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Mix buttermilk, vegetable oil, and vanilla in another bowl. Set aside.Mix butter in mixer bowl at medium speed until creamy. Add sugar slowly and mix on medium speed for 3–5 minutes until lightened and fluffy.
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Add eggs slowly and stir until yolks are absorbed.With the mixer on low speed, alternating flour and buttermilk, starting and ending with flour. After adding, mix well. Do not overmix.
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Split the batter among the three pans.Bake at 350 degrees for 25–30 minutes or until a toothpick inserted in the centre comes out clean or with few crumbs. Turn out after 10 minutes.
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Smoothen the somewhat softened butter. Mix in vanilla.Add half the powdered sugar and most the milk. Add powdered sugar and beat at medium speed.
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Scrape bowl sides occasionally as you mix remaining powdered sugar and milk on medium speed for 3–4 minutes. I slow the mixer. Kitchenaid #2 for 1–2 minutes. Buttercream air pockets shrink. Smoothing will occur.
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Double or halve this recipe.Makes about 4 ½ cups of frosting. Can frost a two-layer 8- or 9-inch cake.Can be frozen for three months in an airtight container.
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Defrost the countertop.Is the buttercream too watery? Put in some sugar. If it is too thick, add milk.
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