Easy Chocolate Sheet Cake with Chocolate Frosting

Preheat the oven to 400 degrees F (200 degrees C). Grease a jelly roll pan with butter and sprinkle with flour.

Mix sugar, flour, and baking soda in a large bowl and set aside.

Combine butter, water, and cocoa powder in a saucepan over medium-high heat and bring to a rapid boil, stirring to make sure it doesn't burn. Pour over the flour mixture and stir to combine. Mix in buttermilk, eggs, and vanilla extract until batter is well combined. Pour batter into the prepared pan.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 18 to 20 minutes.

Melt butter, buttermilk, and cocoa powder in a saucepan over medium-high heat and bring to a rapid boil, stirring to make sure it doesn't burn. Stir in powdered sugar and vanilla extract. Add nuts and stir until frosting starts to lose its gloss.

Spread frosting on the cake while it is still warm.