• Preheat oven to 350°F. Spray a 9x13-inch baking dish* with nonstick cooking spray or line with parchment paper, leaving overhang for easy removal. Set aside.
• In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
• In a separate bowl, whisk together hot coffee, buttermilk, oil, eggs, and vanilla until smooth.
• Pour the wet ingredients into the dry ingredients. Mix gently by hand or on low speed until the batter is smooth and no dry streaks remain. The batter will be thin.
• Pour the batter into the prepared pan and spread evenly with a spatula. Tap pan on the counter to remove air bubbles.
• Bake for 35 to 40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Start checking at 35 minutes. The center may need the full bake time since this is a single thick layer.
• Let the cake cool in the pan on a wire rack for at least 30 minutes, or until completely cool before frosting.

Frosting 

• In a stand mixer with the whisk attachment, or using a hand mixer, combine butter, cocoa, honey, vanilla, and confectioners' sugar.
• Turn mixer on low and let it combine ingredients for about 30 seconds.
• Slowly add milk, one tablespoon at a time, until you reach your desired consistency.
• Spread the frosting over the cool cake.