In a mixing bowl allow egg whites to stand at room temperature for 30 minutes. Meanwhile, grease and lightly flour two 8x1-1/2-inch round baking pans; set pans aside. Stir together flour, baking powder, and 1/4 teaspoon salt; set aside.
In a small heavy saucepan melt the chopped white baking bar with 1/4 cup of the half-and-half over very low heat, stirring constantly until baking bar starts to melt. Immediately remove from heat; stir until baking bar is completely melted and smooth. Stir in remaining half-and-half; cool.
In a large mixing bowl beat the butter or margarine with an electric mixer on medium to high speed about 30 seconds. Add sugar and vanilla; beat until well combined. Add egg yolks, one at a time, beating until combined. Add the dry mixture and the white baking bar mixture alternately to beaten mixture, beating on low to medium speed after each addition just until combined.
Thoroughly wash the beaters. Beat egg whites with an electric mixer on high speed until stiff peaks form (tips stand straight). Gently fold egg whites into the batter. Spread batter in the prepared pans.
Bake in a 350 degree F oven for 25 to 30 minutes or until a wooden toothpick comes out clean. Cool on wire racks for 10 minutes. Remove from pans. Cool thoroughly. Makes 12 servings.