Preheat your oven to 350°F (175°C) and prepare three 8-inch cake pans. Grease the pans and line the bottoms with parchment paper. If you have cake strips, wrap them around the pans and set them aside.
In a medium bowl, whisk together the flour, cornstarch, cocoa powder, baking soda, baking powder, and salt. Set this mixture aside.
In a measuring cup, combine the milk with vinegar and vanilla, then set it aside.
In a large mixing bowl, whisk together the sugars, oil, and mayonnaise. Add the eggs and egg yolk, mixing until well combined.
Gradually add the milk mixture to the sugar mixture, stirring to combine.
Next, pour in half of the dry ingredients and whisk to incorporate.
Add the hot coffee and whisk again to blend everything together, then add the remaining dry ingredients. The batter will be thin and liquid.
Evenly distribute the batter among the three prepared pans.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs.
Allow the cakes to cool in the pans for about 10 minutes, then run a knife around the edges and turn the cakes out onto a cooling rack. Let the cakes cool completely, either at room temperature or in the refrigerator, before frosting.