Preheat Oven & Prepare Pan:

Preheat oven to 350°F (175°C).

Grease and flour a 9×13-inch baking dish.

Bake the Cake:

In a medium saucepan, melt butter with cocoa powder and water. Remove from heat and cool slightly.

In a large bowl, whisk together flour, sugar, baking soda, and salt.

Add melted butter mixture, buttermilk, eggs, and vanilla. Stir until smooth.

Pour batter into prepared pan and bake for 25–30 minutes or until a toothpick inserted in the center comes out clean.

Prepare the Topping: