Preheat oven to 350°F and grease a 9x13 pan. Whisk flour, sugar, cocoa, baking soda, baking powder, and salt. Add buttermilk, oil, eggs, and vanilla, mix until combined, then stir in hot coffee.

Pour batter into the pan and bake 30 to 35 minutes until a toothpick comes out with a few moist crumbs. Cool in pan 10 minutes, then flip onto a rack to cool completely.

Beat butter until fluffy, then add powdered sugar, cocoa, 5 tbsp cream, and vanilla. Beat 3 minutes, adding more cream if needed for a pipeable consistency.

Level the cake top, apply a crumb coat, chill 15 minutes, then frost the entire cake with the remaining chocolate frosting.

Crush cookies into crumbs and press them onto the top of the cake. Tint about two-thirds of the remaining frosting green and pipe a grid of grass tufts. Slice and serve.