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.

Once the cakes are in the oven, start making the frosting.

Using a hand or stand mixer with the paddle attachment, beat the butter until smooth.

Add in the powdered sugar, corn syrup, vanilla, and salt, mixing until fully combined.

In a separate measuring cup, whisk together the warm water and cocoa powder until smooth.

Pour this cocoa mixture into the frosting and mix to combine, scraping down the sides of the bowl as necessary.

Next, pour in the melted chocolate and mix until fully incorporated.

The frosting should be smooth and glossy but somewhat thin. If needed, place the bowl in the refrigerator for about 45 minutes to thicken.

After chilling, stir the frosting by hand with a rubber spatula. If it holds its shape inside the bowl, it's ready to use.

Ensure the cake layers are completely cooled before assembling.

Level the tops of each cake layer, if necessary.

Place the first cake layer on a cake turntable, then spread a layer of frosting on top. Add the next cake layer and repeat the process.

Once all the layers are stacked, refrigerate the cake for about 20 minutes to allow the layers to set.

After the layers are set, cover the entire cake with the remaining frosting.

Now, slice and enjoy!