Prep the pan: Heat the oven to 350°F (175°C). Grease a 13x18-inch rimmed sheet pan (half-sheet) or a 12x17 jelly roll pan. Line with parchment and lightly grease the paper for easy release.

Whisk the dry ingredients: In a large bowl, whisk 2 cups flour, 2 cups sugar, 3/4 cup cocoa, 1 1/2 teaspoons baking powder, 1 1/2 teaspoons baking soda, and 1 teaspoon salt until no streaks remain.

Mix the wet ingredients: In another bowl, whisk 1 cup buttermilk, 1/2 cup neutral oil, 2 large eggs, and 2 teaspoons vanilla until smooth.

Combine and add hot liquid: Stir the wet mixture into the dry just until mostly combined. Slowly pour in 1 cup hot coffee (or hot water), whisking gently. The batter will be thin—that’s good.

Pour and bake: Scrape the batter into the prepared pan. Tap the pan on the counter to pop bubbles. Bake 18–22 minutes, or until a toothpick comes out with a few moist crumbs and the top springs back.

Start the frosting: In a saucepan, melt 1/2 cup (1 stick) unsalted butter over medium heat. Whisk in 1/4 cup cocoa and 1/4 cup milk until smooth and steaming, not boiling.

Finish the frosting: Off heat, whisk in 2 1/2–3 cups powdered sugar, a pinch of salt, and 1 teaspoon vanilla until glossy and pourable. Add more milk a teaspoon at a time if too thick.

Frost while warm: Pour the warm frosting over the warm cake. Tilt the pan or use an offset spatula to spread evenly. Sprinkle with chopped pecans or flake salt if using.

Set and serve: Let the cake cool until the frosting sets, about 30–45 minutes. Slice right in the pan and serve squares slightly warm or at room temperature.