Step 1

Preheat the oven to 350F and prep 3 8″ cake pans. Grease the inside and line the bottom with parchment paper. If you have them, wrap the pans in damp cake strips and set aside.

Step 2

In a medium bowl, whisk together the flour, cornstarch, cocoa powder, baking soda, baking powder, and salt. Set aside.

Step 3

In a measuring glass, add the milk and stir in the vinegar and vanilla. Set aside.

Step 4

In a large mixing bowl, whisk together the sugars, oil, and mayo, followed by the eggs and egg yolk.

Step 5

Mix in the milk mixture.

Step 6

Pour in half of the dry ingredients and whisk to combine.

Step 7

Pour in the hot coffee and whisk to combine, followed by the remaining dry ingredients. The batter should be very thin and liquid.

Step 8

Evenly distribute the batter between the three pans.

Step 9

Bake for 30-35 minutes or until a toothpick in the center comes out clean with a few moist crumbs.

Step 10

Allow the cakes to cool in the pan for about 10 minutes, then run a knife around the edges and turn out onto a cooling rack. Cool the cakes completely before frosting, either at room temperature or in the refrigerator.

Step 11

Right when the cakes are done baking, make the frosting.

Step 12

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

Step 13

Then mix in the powdered sugar, corn syrup, vanilla, and salt.

Step 14

In a measuring glass, whisk together the warm water and cocoa powder.

Step 15

Pour into the frosting and mix to combine, scraping down the sides and bottom of the bowl as needed.

Step 16

Then pour in the melted chocolate and mix to fully combine.

Step 17

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

Step 18

Once chilled, give the frosting a good mix by hand using a rubber spatula. If the frosting holds its shape inside the bowl, it’s ready to go.

Step 19

Before assembling, make sure the cake layers are completely cooled.

Step 20

Level off the top of each cake layer if needed.

Step 21

Lay one layer down on a cake turntable and spread a layer of frosting on top. Lay the next cake layer on top, repeating the process.

Step 22

Once all the layers are stacked, place the cake in the refrigerator for about 20 minutes to allow those layers to set. Once set, cover the entire outside of the cake with frosting.

Step 23

Now slice and enjoy!