First start by preheating the oven to 350°F. Line two 8″ round cake pans with a circle of parchment paper on the bottom but not the sides. Combine the yolks, 1/2 the sour cream and the vanilla in a small bowl, whisk to combine, set aside.
Into a mixer bowl put 1 1/4 cups of the sugar, sift in the flour, and salt. Mix on low speed for 10 seconds to distribute. Add the butter to the flour mixture. Mix on low until the butter is incorporated and the batter looks like a paste. Add the other 1/2 of the sour cream to the flour and butter mixture. Increase the speed to medium and mix about 3 minutes until the batter lightens in texture. Scrape the sides and bottom of the bowl. With the mixer on low, add 1/2 the egg yolk mixture. Mix until mostly incorporated. Scrape the sides and bottom of the bowl. Add the rest of the egg yolk mixture. Scrape the bowl and mix until incorporated.
In another bowl, whip the egg whites on medium high until they form soft peaks. Turn the mixer to medium low and slowly add the remaining 1/4 cup of sugar. Turn the mixer to medium high and whip the whites to full peak. Fold the whites into the base in 2 parts, folding just until there are no streaks of egg whites.
Divide the batter between the 2 pans and spread so it’s level. Bake until the center of the cake springs back when lightly pressed or a toothpick inserted into the middle of the cake comes out clean, about 25-30 minutes. Cool for 10 minutes in the pan and then turn out onto a cooling rack. When fully cooked, wrap in plastic and refrigerate at least 3 hours or overnight before filling and assembling the cake.
For the buttercream frosting, blend 1 cup salted margarine with 4 cups icing sugar, ¼ cup milk and 2 tsp. vanilla extract until blended. Beat on medium-high speed, scraping sides occasionally, until light and fluffy, about 2 minutes.