Line a baking sheet with parchment. Trace two 8-in. circles 1 in. apart on paper; set aside. Place egg whites in a large bowl; let stand at room temperature for 30 minutes.
Add cream of tartar; beat until soft peaks form. Sift confectioners' sugar and cocoa together 2 times; gently fold into batter. Fold in vanilla. Spread evenly over circles.
Bake at 275° for 1 hour. Turn oven off and do not open door; let meringues dry for 1 hour.
Remove from oven; cool on baking sheet. When completely cool, remove meringues from paper; store in an airtight container.
In a large bowl, cream butter and sugar until light and fluffy. Add egg, then egg white, one at a time, beating well after each addition. Beat on high speed until light and fluffy. Beat in vinegar and vanilla. Combine the flour, cocoa and baking soda; add to batter alternately with buttermilk and water.
Pour into two 9-in. round baking pans coated with cooking spray and floured. Bake at 350° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks.
Place one cake layer on a serving plate; spread 1/2 cup whipped topping over 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.
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