Combine 1 cup sugar, 1 cup milk, egg yolks, and cocoa in a saucepan: bring to a boil, stirring constantly until thickened. Remove from heat; cool.
Cream butter; gradually add 2 cups sugar, beating well. Add eggs, one at a time, beating well after each addition.
Combine flour and soda; add to creamed mixture alternately with 1 cup milk, beginning and ending with flour mixture. Mix well after each addition. Stir in cocoa mixture and vanilla.
Pour batter into 3 greased and floured 9-inch round cake pans. Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out clean. Let cool in pans 10 minutes ; remove layers from pans, and let cool completely.
Spread Fluffy Filling between layers and on top of cake.