Preheat your oven to 325°F. Prepare a 10×15-inch jelly roll pan by lining it with parchment paper and giving it a light spray of nonstick cooking spray for easy release.

Whisk together the dry ingredients in a large bowl. Combine the all-purpose flour, granulated sugar, baking soda, baking powder, cocoa powder, and kosher salt until evenly mixed.

Mix the wet ingredients in another bowl. In a medium bowl, whisk together mayo, eggs, buttermilk, vanilla extract, red gel food coloring, hot water, and vinegar until smooth and creamy.

Combine wet and dry ingredients. Gradually add the wet mixture to the dry ingredients, stirring gently until just combined. Avoid overmixing, as this helps maintain a tender crumb.

Pour the batter into the prepared pan. Spread it evenly, making sure to smooth the top. Bake for 35-40 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Cool the cake. Transfer it to a wire rack and allow it to cool to room temperature. This ensures that the cake sets properly for frosting later on.

Chill the cake for easy handling. Place it in the freezer for about 10-15 minutes or refrigerate (covered) for at least 1 hour, or overnight if desired, to make moving it simpler.

Prepare the ermine frosting while the cake cools. This step is critical for adding that creamy goodness on top.