Heat 3/4 cup milk in double boiler, add 1 1/2 tablespoons butter. Beat 3 eggs until thick and foamy, gradually add 1 1/2 cups sugar, beating constantly until well blended. Add 1/2 teaspoon salt and 1 1/2 teaspoon vanilla. Beat 4 minutes. Sift together 1 1/2 cups flour, 1 1/2 teaspoon baking powder, quickly add to the egg mixture and stir gently until dry ingredients are just mixed. Stir hot milk mixture into the batter. Pour into a 13 x 9 x 2 pan lined with waxed paper. Bake 30 minutes at 350 degrees.