Preheat oven to 350. Grease 11x7x1-1/2 inch glass baking dish with shortening. Line with waxed paper. Grease waxed paper with shortening.
For cake: combine cake flour, granulated sugar and 1/2 cup shortening in large bowl. Beat at low, then medium speed of electric mixer until crumbly.
Place egg whites in 1-cup measure. Add enough water to measure 1 cup. Pour into crumb mixture. Beat at high speed 10 minutes or until thickened. Add baking powder and vanilla. Beat just until blended.
Spoon 1 cup batter into small bowl. Add peanut brittle and sundae topping. Stir until blended. Spoon remaining batter into prepared baking dish. Place spoonfuls of peanut brittle batter on top. Cut through batters with knife for marble effect.
Bake at 350 for 30 to 35 minutes or until top springs back when touched lightly in center. Cool 5 minutes before removing from baking dish. Invert cake onto wire rack. Remove waxed paper. Cool completely. Place cake on serving tray.
For frosting: combine milk and all-purpose flour in small saucepan. Stir until blended. Cook and stir over medium heat until mixture thickens. Transfer to medium bowl. Cool completely. Add confectioners sugar, shortening and vanilla. Beat at low speed until blended. Beat at medium speed 4 to 5 minutes. Frost top and sides of cake.
For topping: drizzle top of cake with additional sundae topping; letting excess topping run down sides. Sprinkle top of cake with additional crushed peanut brittle.