Grease a 1"x 10"x 15" cookie sheet and line with wax paper. Grease and flour wax paper. Beat egg whites with salt until they hold soft peaks. Gradually beat in sugar, sprinkling it in 1 tablespoon at a time. Continue beating until whites are firm about 5 minutes. Stir yolks with a fork to break them up. Add vanilla, fold 1/4 of the stiffly beaten egg whites thoroughly into egg yolk, then fold remaining egg whites. Sprinkle cornstarch and flour on top, fold very gently together until egg white is blended. Be careful not to overmix. Pour into prepared pan. Spread batter evenly, bake in preheated oven 400 degrees, 10-12 minutes or until cake is very lightly browned. Be careful not to overbake. Loosen sides and remove cake from pan at once. Cool on rack before peeling off paper. Cake should be flexible from end to end. Cut cake in half, then split each layer to make 3 layers. Sprinkle each layer with rum and top with the filling, alternate the white cream filling with the chocolate. Melt chocolate in a double boiler or microwave oven. Cool chocolate slightly then cover the top and the sides of cake. Refrigerate.
Butter Cream Filling:
Put eggs and sugar on top of the stove to warm up, then beat the mixture in a blender, adding the butter a little at a time to a pudding consistency. Add vanilla. Divide cream in thirds. Use white cream filling for two layers of the cake. Add cocoa to remaining cream and spread on third layer.