First, Make The Cake:

1. 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.
2. 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.
3. Combine Dry Ingredients. In a medium bowl, combine gluten-free flour blend, cornstarch, baking powder, and salt. Whisk to combine, then set aside.
4. 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.
5. 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. 
6. Mix In Sour Cream. Scrape down the sides of the bowl again, and mix in the sour cream on LOW speed.
7. 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!)
8. 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.
9. 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.)
10. 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.

When The Cake Is COMPLETELY Cool, Make The Frosting***

1. Beat The Butter. Add the softened butter to a large bowl. Beat with a hand mixer on low speed for about 2 minutes, until it’s light and fluffy.
2. Mix. Then, add powdered sugar, cocoa powder, salt, and vanilla extract. Add the cream, then mix for 30 seconds on low speed, then about 1 minute on medium speed, or until well combined.
3. Adjust. At this point, you’ll want to adjust for texture. If the frosting is too dry or thick, add a little bit of additional cream at a time until it thins out. If the frosting is too thin or too wet, you can add more powdered sugar or cocoa powder a teaspoon at a time, until you reach the right consistency. The frosting should be easy to spread but should still hold its shape. (If you want the frosting to look extra glossy, you can add 1-2 Tablespoons of corn syrup at this point.)

Assemble The Cake:

1. Stack The. Layers. When you’re ready to assemble the cake, place the base cake layer so the bottom of the cake (what was in the bottom of the cake pan) is now on the top. This will give you the most level, neat surface to work with. Add about 2/3-1 cup frosting and use an offset spatula to smooth the frosting into an even layer, all the way to the edges. It’s ok if it spills over the sides. (We’ll smooth this down later.) Add the second layer of cake (again putting side side that was in the bottom of the cake pan face up.) Add another 1 cup or so of frosting to the top of the cake.

2. Frost. Smooth the frosting out with an offset spatula, then smooth or swoop the rest of the frosting along the sides of the cake, until the whole cake is covered. This recipe for frosting will be enough to just cover the outside and inside of the cake. (If you’d like to decorate or add additional details, you can 1.5x the frosting.)

3. Decorate. If desired, you add sprinkles or you can pipe a border or decorations with any remaining or additional frosting. (I like to use a Wilton 2D or Ateco 849 closed star tip to pipe a spiral or shell border or to make little poofs of frosting around the top of the cake)

4. Serve & Store. Enjoy the cake right away, or cover the cake and refrigerate until you’re ready to serve. (Cake will slice more neatly if chilled.) Store leftover cake covered in the refrigerator 2-3 days, or freeze leftovers up to 2 months.