Line a baking sheet with parchment paper; trace two 8-inch circles 1 inch apart on the paper; set aside.
In a mixing bowl, beat egg whites and cream of tartar until soft peaks form. Add sugar one tablespoon at a time, beating until stiff peaks form. Sift confectioners' sugar and cocoa over meringue; gently fold inches Fold in vanilla. Spread evenly over circles. Bake at 275°F for 1 hour.
Turn oven off and do not open door; Let meringues dry for 1 hour. Remove meringues from paper and store in an airtight container.
In a mixing bowl, cream butter and sugar for 2 minutes. Add egg, then egg white, one at a time, beating well after each addition. Beat on high speed until light and fluffy. Stir in vinegar and vanilla. Combine flour, cocoa, and baking soda; add to batter alternately with buttermilk and water. Pour into two 9-inch round baking pans coated with nonstick cooking spray and floured. Bake at 350°F for 20-25 minutes or until a toothpick comes out clean. Cool for 10 minutes before removing from pans to wire racks.
To assemble, spread 1/2 cup whipped topping over one cake layer; top with a meringue and another 1/2 cup topping. Top with second cake layer, 1/2 cup topping, second meringue, and remaining topping. Sprinkle with toffee bits and shaved chocolate.