Dark Chocolate Sheet Cake with Dark Chocolate Frosting

Preheat the oven to 350 degrees F (175 degrees C).

Grease a 10x15-inch rimmed baking sheet and line the bottom of the pan with parchment paper.

Whisk flour, cocoa powder, baking soda, and salt together in a bowl until thoroughly combined.

Beat brown sugar, white sugar, and butter together in a large bowl with an electric mixer until light and fluffy.

Beat in eggs one at a time, mixing well after each addition.

Mix in vanilla extract.

Add in 1/3 of the flour mixture, and mix until just combined.

Add in 1/2 of the coffee and buttermilk, and mix until just combined.

Continue alternating additions of the flour mixture with coffee and buttermilk, mixing just until combined.

Pour batter into the prepared pan; smooth into an even layer.

Tap pan firmly on the counter several times to remove air bubbles.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out with a few moist crumbs, about 25 minutes.

Do not overbake.

Place cake on a wire rack and allow to cool completely.

Meanwhile, make the frosting.

Beat butter in a large bowl until completely smooth.

Mix in cocoa powder, espresso powder, and salt until thoroughly combined.

Mix in vanilla.

Add in powdered sugar 1 cup at a time, mixing well after each addition.

Pour in heavy cream and beat until frosting is light and fluffy, 2 to 3 minutes.

If frosting is too thick, add more heavy cream.

Spread frosting over the cooled cake.