Preheat oven to 350 degrees. Grease two 9" cake pans with butter then set aside.

In a small bowl combine egg yolks, ¼ cup sour cream and vanilla. Whisk to combine.

Combine flour, cornstarch, sugar, baking soda and salt in the bowl of a stand mixer fitted with the paddle attachment. Beat on high for 30 seconds to combine. NOTE: We strongly recommend you weigh your einkorn flour, if possible, for better accuracy. If you don’t have a scale, fluff the flour with a fork before you measure it in a cup.

Add softened butter and remaining ½ cup of sour cream. Use a few turns of the paddle to moisten the mixture, scraping down the sides as needed. Do not over-mix! When completely moistened, add yolk mixture in three batches. Mix only until incorporated. Transfer to a large bowl. Wash and dry the mixer bowl. It must be clean and dry to whip egg whites.

Reattach the clean and dry bowl to the mixer. Using the whisk attachment, beat reserved egg whites plus two more egg whites (for a total of 8 egg whites) for one minute until foamy. Add cream of tartar and turn on high. Beat the whites until a stiff peak forms. Fold whites into the batter in three batches using a rubber spatula, being careful to keep as much air in the mixture as possible.

Divide batter between two buttered pans then bake for 35–40 minutes until a toothpick inserted into the center comes out with just a few crumbs.

Transfer cakes to a rack and let cool for 10 minutes. Run a knife around the sides of each pan to loosen and remove the cakes from the pans. Let cool completely—about 1 hour.

When the cakes are completely cool, place a cardboard cake round on a lazy susan or pizza pan, directly in the center. You will be rotating the cake, so make sure it’s centered. It’s easier if it is off the surface of the counter (use an inverted bowl or your cake stand) but not essential.

First, level your cakes. Place a cake layer, rounded side up, on the cake round. Hold a serrated knife with the sharp side toward the cake and perfectly level at the edge of the cake where it starts to round higher. Do not move the knife! Instead, rotate the cake with your other hand. As you turn the cake, it will slice into the cake all the way around until you are back where you started. If you were holding your knife level, your knife will slide right into the cut you started with. Now start moving the knife toward the center, all while you are still rotating the cake. Very soon you will have cut all the way through the cake. Lift off the cut part and you will have a flat cake layer! Repeat with the other layer or layers and set the cake aside. The tops will be a great snack later with some pudding or ice cream.

Drop a dollop of the cream mixture in the center of the cardboard round and place one of the cakes on the round. The cream will anchor the cake. Spoon half of the cream mixture onto the cake. It will seem like a lot but you are creating a thick layer of cream between the cakes. Using an offset spatula, spread cream perfectly flat and slightly past the edge of the cake. Place the second cake on top with the bottom of the cake facing up. Spin cake to be sure it is perfectly centered. Using a butter knife or small offset knife, smooth the sides of the cake.

Add most of the remaining cream to the top of the cake, saving a bit to fix any mistakes. Spread perfectly flat and smooth the sides. It will look like there are four layers. Cake, filling, cake, filling starting from the bottom. Store in a cool place while you prepare the ganache, avoiding the refrigerator so it does not dry out.