• Preheat oven to 325°F. Spray a 9×13-inch baking dish with nonstick cooking spray. Line with parchment if desired.
• In a large bowl, whisk together the flour, sugar, baking soda, baking powder, cocoa powder, and salt.
• In a separate bowl, whisk together the mayonnaise, eggs, buttermilk, vanilla, red food coloring, hot water, and vinegar until smooth.
• Add the wet ingredients to the dry ingredients and mix until just combined.
• Pour the batter into the prepared pan and smooth the top. Tap the pan 2 to 3 times on the counter to release air bubbles.
• Bake for 38 to 43 minutes, or until a toothpick inserted in the center comes out with a few crumbs but no wet batter.
• Let the cake cool completely in the pan before frosting.

Ermine Frosting

• In a medium saucepan over medium heat, whisk together the milk and flour. Cook, whisking constantly, for 3 to 5 minutes, or until no lumps remain and the mixture thickens and resembles a very thick pudding. It should coat the back of a spoon.
• Remove the pan from the heat and let the mixture cool to room temperature.
• In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium speed until light and fluffy (2 to 3 minutes). Stop the mixer and use a rubber spatula to scrape down the sides of the bowl.
• Add the cooled milk mixture to the bowl of the stand mixer. Beat on medium-high speed until the frosting is light and fluffy (2 to 3 minutes), pausing to scrape down the sides of the bowl as needed.
• Switch to the whisk attachment. Add vanilla and mix on high for 7 to 8 minutes, or until the frosting is smooth and creamy.
• Spread the frosting evenly over the cooled cake.