Heat oven to 350ºF (177ºC) with rack set in middle position. Grease and flour a 15x10-inch jellyroll pan.
Whisk together 2 cups flour, 2 cups sugar and 2 teaspoons baking soda in a large bowl; set aside.
Add 1 cup butter, 1 cup water, ¼ cup cocoa powder and a pinch of salt to a saucepan; bring to a boil over medium heat, whisking occasionally to combine. Pour mixture over dry ingredients and stir until combined.
Add ⅔ cup buttermilk, 2 eggs, and 1 teaspoon vanilla; mix to combine. Pour batter into prepared pan and spread into an even layer using an offset spatula.
Bake until a toothpick pick inserted in center comes out clean, about 20 minutes. Transfer to a wire rack and cool completely.
Crème de Menthe Frosting
Using an electric mixer, beat together ½ cup butter and 3 ounces cream cheese until smooth. Add 2 tablespoons pudding mix and beat to combine. Gradually add 2 cups powdered sugar, beating until light and fluffy.
Stir in 1½ tablespoons crème de menthe and a pinch of salt.
Spread frosting over cooled cake; chill until frosting is set.
Storage:Wrap the pan tightly in plastic wrap and keep it in the fridge for up to 2 weeks.Sifting the powdered sugar:this may seem like an unnecessary step, but sifting breaks up any lumps and makes for a very smooth and lump-free frosting.This recipe is developed and intended to be baked in a jellyroll pan (15x10-inch pan). A half sheet pan (18x13-inch) will be too large. If you want to use a half sheet pan, I recommend doubling the recipe with the knowledge that some of the batter may end up getting thrown away.
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