Preheat your oven to 350°F (175°C) and grease a jelly roll pan.

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

In another bowl, mix buttermilk, eggs, and vanilla extract until smooth.

Melt butter, cocoa powder, and water in a saucepan over medium heat until combined.

Combine the hot chocolate mixture with the dry ingredients, then fold in the wet ingredients.

Pour the batter into the greased jelly roll pan and bake for 15-20 minutes.

Prepare frosting by melting butter, cocoa powder, and buttermilk, then whisk in sugar and vanilla.

Pour the warm frosting over the cooled cake.

Let the cake cool slightly before serving.