Preheat oven to 375°. Beat butter and 2 cups sugar together until light and creamy. Beat in vanilla. Sift together flour, baking powder and 1/4 tsp. salt twice. Stir flour mixture into batter alternately with milk. Beat egg whites until stiff but not dry. Stir 1/4 of whites into batter. Fold in remaining whites until just mixed. Spoon into 3 greased and floured 9-inch pans and bake 20 to 25 minutes. Cool in pans for 10 minutes; turn onto cake racks.

To prepare filling, mix together yolks, 1 1/4 cup sugar and orange rind in a heavy pan or in top of a double boiler. Cook over medium heat, stirring constantly until sugar dissolves and mixture thickens enough to coat back of spoon. Do not allow to boil or eggs will scramble. Remove from heat and stir in remaining ingredients. Cool before using to fill and top the cake layers.