• Preheat oven to 350 degrees F. Spray a 13 by 9-inch baking dish with non-stick cooking spray.
• In a medium mixing bowl whisk together flour, cocoa powder, baking soda and baking powder for 20 seconds, set aside.
• In a separate large mixing bowl, using an electric hand mixer set on high speed, beat eggs, sugar and vanilla for 3 minutes. Blend in mayonnaise.
• Mix half of the flour mixture into the sugar/mayonnaise mixture, mix in the water then mix in remaining flour mixture. Pour into prepared pan. 
• Bake in preheated oven until toothpick inserted into center comes out clean, about 34 - 38 minutes. Let cool completely on a wire rack (I like to cover about halfway through to seal in moisture). 
• Frost completely cooled cake with marshmallow frosting then top with chocolate shavings. 

For the frosting:

• Fill a large saucepan with about 1 1/2-inches of water and bring to a simmer. Add egg whites, granulated sugar and cream of tartar to a heat proof bowl of an electric stand mixer.
• Set bowl over saucepan (make sure it doesn't touch water) and heat mixture, whisking constantly until it's very warm (I let mine get to 150 degrees F), about 3 - 4 minutes. 
• Remove from heat and transfer bowl to electric stand mixer fitted with a whisk attachment. 
• Beat mixture beginning on low and increasing to high speed until stiff glossy peaks form, about 5 - 7 minutes. 
• Pour in vanilla during the last 10 seconds of mixing. For best results serve within a few hours of preparing - keep frosting cold.