1Make the cake according to package directions. Bake in a 13x9 pan as directed on package.
Spread butterscotch filling (below) over top of cooled cake to within 1/2 inch of edge. Sprinkle 1/2 cup chopped nuts over filling.
2BUTTERSCOTCH FILLING: In medium saucepan, mix brown sugar & flour. Gradually stir in the cream. Heat to boiling, stirring constantly. Boil and stir 1 minute. Remove from heat; blend in butter and vanilla. Cool.
3MARSHMALLOW FROSTING: In top of double boiler, combine egg whites, sugar, cream of tartar, syrup and water. Place over boiling water, beat until stiff peaks form, scraping pan occasionally. Remove from heat; add marshmallow creme. Beat until of spreading consistency.
4Remove cake to cake platter, if desired. Frost sides and top of cake with Marshmallow Frosting. Melt 1/2 square unsweetened chocolate; dip back of spoon into chocolate and form swirls on frosting.