Beat margarine at medium speed of an electric mixer until creamy; add 1 1/2 cups sugar, beating until fluffy. Add egg whites and egg, one at a time, beating after each addition. Combine buttermilk and water. Combine flour and next 3 ingredients; add to margarine mixture alternately with buttermilk mixture. Mix after each addition.
Pour batter into 2 (8-inch) round cakepans coated with cooking spray. Bake at 350° for 22 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans. Let cool.
Stir raspberry spread well. Slice each cake layer in half horizontally. Place 1 layer on plate; spread with 1/4 cup raspberry spread. Repeat with next 2 layers; top with fourth layer. Cover and chill.
Combine remaining 1/2 cup sugar, 2/3 cup cocoa, and cornstarch in top of a double boiler. Stir in milk. Bring water to a boil. Reduce heat to low; cook, stirring constantly, 18 minutes or until of spreading consistency. Stir in vanilla. Cover and chill. Spread on top and sides of cake. If desired, garnish with raspberries and mint sprigs.