How to Make It
To make the cake, bring the water to a boil in a medium saucepan. Add the butter to the water and stir in the cocoa powder until smooth. Remove from the heat and cool slightly. Add the buttermilk, eggs, and vanilla extract; stir until well combined.
Add the flour, soda, and salt and whisk until smooth.
Place a rack in the center of the oven; preheat oven to 350 °. Spray a 9x13-inch sheet pan (a half-sheet pan) with nonstick cooking spray and line with parchment paper. Pour the batter into the pan and bake for 15 minutes. Allow to cool before frosting.
To make the frosting, mix each whipped topping with a separate flavor and spread generously in thirds across the cooled cake.