• Preheat oven to 325°F. Line a 10×15-inch jelly roll pan (a rimmed baking sheet with 1-inch sides) with parchment paper, making sure the paper comes up the sides. Spray the parchment with nonstick cooking spray and set aside.
• In a large bowl, whisk together the flour, sugar, baking soda, baking powder, cocoa powder, and kosher salt.
• In a medium bowl, whisk together the mayonnaise, eggs, buttermilk, vanilla extract, red gel food coloring, hot water, and vinegar until smooth.
• Slowly add the wet ingredients to the dry ingredients, stirring with a spatula or spoon until just combined and no dry streaks remain. Do not overmix.
• Pour the batter into the prepared pan, smoothing the top. Bake for 35-40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs, but no wet batter.
• Place the pan on a wire rack and let the cake cool to room temperature.
• Transfer the cooled cake to the freezer for 10–15 minutes until firm, or refrigerate (covered) for at least 1 hour or up to overnight. Chilling the cake makes it easier to move.
• While the cake chills, prepare the ermine frosting.

Frosting

• In a medium saucepan over medium heat, whisk together the whole milk and flour. Cook, whisking constantly, for 3–5 minutes until thick and smooth, resembling a very thick pudding and coating the back of a spoon.
• Remove from heat and let cool completely 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, about 2–3 minutes. Scrape down the sides of the bowl with a rubber spatula.
• Add the cooled milk-flour mixture to the mixer bowl. Beat on medium-high speed until light and fluffy, about 2–3 minutes, scraping down the bowl as needed.
• Switch to the whisk attachment. Add the vanilla extract and whisk on high speed for 7–8 minutes, or until the frosting is smooth and creamy.

Assembly

• Remove the chilled cake from the freezer or refrigerator. Using the parchment edges, lift the cake from the pan and place it on a work surface.
• Cut the cake in half (widthwise) to make two equal rectangles.
• Carefully place one rectangle onto a serving platter. Spread half of the ermine frosting over the cake layer.
• Top with the second cake rectangle. Spread the remaining frosting over the top.
• Slice and serve immediately.