Step 1

Preheat the oven to 350°F. Spray a 10x15-inch sheet pan (jelly roll pan) with nonstick cooking spray.

Step 2

To the bowl of a stand mixer, add the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Mix to combine.

Step 3

In a separate bowl, combine the buttermilk, oil, eggs, and vanilla.

Step 4

With the mixer on low speed, slowly add the wet ingredients to the dry. With the mixer still on low, add the coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula.

Step 5

Pour the batter into the prepared pan and bake for 28-30 minutes, or until a cake tester or toothpick comes out mostly clean with no wet batter.

Step 6

Let the cake cool in the pan completely. Then, place it in the freezer or refrigerator until firm.

Step 7

In a medium saucepan over medium heat, whisk together the milk and flour. Cook, whisking constantly, for 3-5 minutes, or until no lumps remain and the mixture thickens and resembles a very thick pudding. It should coat the back of a spoon.

Step 8

Remove the pan from the heat and let the mixture cool to room temperature.

Step 9

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium speed until light and fluffy (2-3 minutes). Stop the mixer and use a rubber spatula to scrape down the sides of the bowl.

Step 10

Add the cooled milk mixture to the bowl of the stand mixer. Beat on medium-high speed until the frosting is light and fluffy (2-3 minutes), pausing to scrape down the sides of the bowl as needed.

Step 11

Switch to the whisk attachment. Add vanilla and mix on high for 7-8 minutes, or until the frosting is smooth and creamy.

Step 12

In a medium saucepan over medium heat, whisk together the milk and flour. Cook, whisking constantly, for 3-5 minutes, or until no lumps remain and the mixture thickens and resembles a very thick pudding. It should coat the back of a spoon.

Step 13

Remove the pan from the heat and let the mixture cool to room temperature. (The mixture will get lumpy again as it cools.)

Step 14

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, sugar, and cocoa powder on low speed until the cocoa powder is incorporated. Increase the speed to medium and mix until light and fluffy (2-3 minutes). Stop the mixer and use a rubber spatula to scrape down the sides of the bowl as needed.

Step 15

Add the cooled milk mixture to the bowl of the stand mixer. Beat on medium-high speed until the frosting is light and fluffy (2-3 minutes), pausing to scrape down the sides of the bowl as needed.

Step 16

Switch to the whisk attachment. Add vanilla and mix on high for 8-10 minutes, or until the frosting is smooth and creamy.

Step 17

Once the cake has cooled completely and been chilled, slice it in half down the middle. You should have two rectangles. Gently place one of the cakes onto a serving plate.

Step 18

Spread the vanilla frosting over one of the halves of the cake.

Step 19

Place the other cake layer over the vanilla frosting. Top with the chocolate frosting. Enjoy!