Preheat oven to 325°F. Line a 10×15-inch sheet pan with parchment paper and spray with nonstick spray.

In a large bowl, whisk together flour, sugar, baking soda, baking powder, cocoa powder, and salt.

In a medium bowl, whisk together mayonnaise, eggs, buttermilk, vanilla, food coloring, hot water, and vinegar until smooth.

Combine wet and dry ingredients just until combined. Pour batter into prepared pan and bake for 35-40 minutes.

Let the cake cool on a wire rack. Prepare the frosting while it cools.

Make the frosting: whisk milk and flour in a saucepan over medium heat until thickened. Cool completely.

In a mixer, beat butter and sugar until fluffy. Add cooled milk-flour mixture and vanilla; beat until smooth.

Assemble the cake: cut cooled cake into slices, layer with frosting, and serve.