Step 1

Preheat the oven to 350 degrees F.

Step 2

Brush the bottoms and sides of two 9-inch round cake pans with the extra vegetable oil and dust with the extra flour, tapping out the excess.

Step 3

In a medium bowl, whisk the remaining 2 cups of flour, 1 cup of the cocoa powder, 1 teaspoon of the salt, the baking soda, and the baking powder together.

Step 4

In a separate medium bowl, add 2 ounces of the chopped chocolate and the instant coffee.

Step 5

Pour the boiling water over the chocolate-coffee mixture and let it stand for 5 minutes.

Step 6

Whisk the chocolate-coffee mixture until it is smooth. Set it aside.

Step 7

In a liquid measuring cup, add the sour cream, the remaining 1/2 cup vegetable oil, and the vanilla and stir to combine; the mixture will look curdled. Set it aside.

Step 8

In a large bowl, add 3 of the eggs and 2 cups of the sugar and beat with an electric hand mixture on medium speed until combined.

Step 9

Increase the speed to medium-high and beat the egg mixture until it is light and fluffy, about 4-5 minutes.

Step 10

Reduce the speed to low. Slowly add the chocolate-coffee mixture to the egg mixture and beat until it is combined.

Step 11

In three batches, alternate beating the flour mixture and the sour cream mixture into the batter mixture until it is combined and resembles hot fudge. After each addition, increase the speed to medium, then return to low as the ingredients are being added.

Step 12

Divide the batter between the two prepared pans.

Step 13

Bake the cakes until a toothpick inserted into the center comes out clean, about 35-45 minutes.

Step 14

Let the cakes cool in the pans for 15 minutes.

Step 15

Run a knife around the edges of the cakes, then invert them onto a wire rack to cool completely.

Step 16

While the cakes are cooling, in a medium bowl, whisk the remaining eggs, 1/4 cup of the milk, the remaining sugar, the remaining cocoa powder, the cornstarch, and the remaining salt together until it is thick and smooth.

Step 17

In a saucepan over medium heat, warm the remaining milk until it is just steaming.

Step 18

Gradually whisk the hot milk into the cocoa mixture.

Step 19

Pour the frosting mixture into the same saucepan and bring to a boil, whisking constantly.

Step 20

Cook the frosting mixture, whisking vigorously, until it thickens, about 30-60 seconds.

Step 21

Transfer the frosting mixture from the heat and pour it into a heat-safe bowl.

Step 22

Press a piece of plastic wrap directly onto the surface of the frosting mixture and refrigerate until it is cold, about 3-4 hours.

Step 23

Put the remaining chocolate in a microwave-safe bowl and microwave it in intervals until it is melted, for 30 seconds at a time.

Step 24

Let the melted chocolate cool slightly.

Step 25

In a large bowl, add the remaining butter and beat with an electric hand mixer on medium-high speed until it is light and creamy, about 3-5 minutes.

Step 26

Stir the frosting mixture to loosen it.

Step 27

Gradually add the frosting mixture into the melted butter and beat well after each addition.

Step 28

Beat the melted chocolate into the frosting mixture, scraping down the sides of the bowl as needed, until it is fluffy, about 1-2 minutes.

Step 29

Use a serrated knife to trim off the domed tops of the cakes.

Step 30

Put one of the cakes on a platter.

Step 31

Spread 1 cup of the frosting over the top of the cake, then place the remaining cake on top.

Step 32

Spread the remaining frosting over the top and down the sides of the cakes.

Step 33

Refrigerate the cake for 1 hour.

Step 34

Slice and serve.