Mix 1 T. cocoa with 3 T sugar in small bowl. Line bottoms of three 8-inch layer pans with wax paper, spray with non-cooking spray and dust sides of pan with cocoa/sugar mixture.
In a saucepan over medium low heat, combine 3/4 cup cocoa, 3/4 cup sugar, 1 egg yolk, and 1/2 cup milk. Cook, stirring constantly to prevent sticking, until thickened. Cool.
Cream butter until light and fluffy; add remaining 1 cup of sugar, beating until well blended. Add 1 egg and 2 egg yolks; mix well. Stir in sifted dry ingredients alternately with sour cream and mayonaise. Add vanilla and cooled cocoa mixture. Beat remaining (3) egg whites until stiff; fold into cake batter. Pour batter evenly into the three pans.
Bake for 30 to 35 minutes, or until a cake tester or wooden toothpick inserted in the center comes out clean. Let cool in the pans for about 20 minutes. Invert onto wire racks, carefully peel off the paper liners, and let cool completelybefore frosting. I found them a lot easier to work with after firming them up in the freezer for 30 minutes. They’ll defrost quickly once assembled. You’ll be glad you did this, trust me.).
Melt chocolate with butter in Microwavable bowl for 1 minute, stir to combine; stir in sour Cream, vanilla and salt. I pour this mixture into my stand mixer then slowly beat in powdered sugar Until desired spreading consistency.
To frost the cake, place one layer, flat side up, on a cake stand or large serving plate. Spread 2/3 cup frosting evenly over the top. Repeat with the next layer. Place the last layer on top and frost the top and sides of the cake with the remaining frosting.
Making a crumb coat of frosting–a thin layer that binds the dark crumbs to the cake so they don’t show up in the final outer frosting layer–is a great idea for this cake, or any with a dark cake and lighter-colored frosting. Once you “mask” your cake, let it chill for 15 to 30 minutes until firm, then use the remainder of the frosting to create a smooth final coating. Once the cake is fully frosted, it helps to chill it again about 30 minutes to let it firm up.