Preheat the oven to 350°F (175°C).

Grease the sheet cake pan with butter or cooking spray.

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

In a saucepan, melt the butter with the water over medium heat until bubbling.

Remove from heat and whisk in the cocoa powder until smooth, then gradually stir in the buttermilk, eggs, and vanilla extract.

Pour the wet mixture into the dry ingredients and mix until just combined.

Pour the batter into the prepared pan and bake for 20 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for about 10 minutes and frost with your favorite chocolate frosting while still warm.