Set Out Ingredients In Advance. You MUST set out the cold ingredients (eggs, butter, milk, and sour cream) about 1-1.5 hours in advance to let them come up to room temperature. Otherwise, they will not cream properly and the batter will be chunky instead of smooth.
Preheat The Oven & Prep Your Pans. Start by preheating the oven to 325 degrees F. Grease two 8-inch round cake pans and line them with parchment paper.
Combine Dry Ingredients. In a medium bowl, combine gluten-free flour blend, cornstarch, baking powder, and salt. Whisk to combine, then set aside.
Cream Butter & Sugar. In the bowl of a stand mixer, combine softened butter and sugar. Use the paddle attachment and beat together 2-3 minutes on medium-high speed until light and fluffy.
Add Eggs & Vanilla. Scrape down the edges/bottom of the bowl and gradually add eggs one at a time, then the vanilla, mixing to incorporate well.
Mix In Sour Cream. Scrape down the sides of the bowl again, and mix in the sour cream on LOW speed.
Alternate Dry Ingredients & Milk. Add half of the dry ingredients and mix on LOW speed, streaming in half of the milk as you go. Scrape down the edges of the bowl and add the second half of the dry ingredients, streaming in the second half of the milk until just combined. Try not to over-mix here. Feel free to finish combining by hand with a spatula, if needed!)
Pour Batter Into Pans. Carefully divide the cake batter between the two prepared pans. Use an offset spatula to even out the surface of the batter.
Bake Cake Layers. Bake both layers on the same shelf in the oven for 35-40 minutes or until the top of the cake springs back when gently pressed, and a cake tester inserted in the center comes out clean with just a few moist crumbs. (The finished cake's internal temperature should be 205 degrees.)
Cool Completely Before Frosting. Let the cake layers cool 15 minutes in the pans before carefully turning them out onto a wire rack to cool completely. Note that the cake must be COMPLETELY cool before frosting or the frosting will melt.