To make the Cakes - In a large bowl, mix the first 4 ingredients. Make a well and add the next 4 ingredients. Beat until smooth with a spoon and set aside. Beat the egg whites with the cream of tartar until stiff peaks form. Pour over egg yolk batter and fold in until well beaten. Pour in an ungreased 10x14 pan. Bake 45-50 minutes. Cool in pan. Invert in pan with cups under each corner. Cool completely and run a knife around the edge. Remove and cut in half.
To make the Filling - Combine the first 3 ingredients and beat on high for 5 minutes, gradually adding the milk and vanilla. Beat 5 more minutes on high. Spread between 2 layers. Cut to size (3x1 inch) and wrap separately.