Preheat the oven to 350°F (175°C) and grease a 10x15‑inch sheet pan.

Whisk together the flour, granulated sugar, and baking soda in a large bowl.

In a saucepan melt the butter, then stir in cocoa powder and milk/water; bring to a boil and remove from heat.

Add the melted mixture to the dry ingredients, then fold in the eggs and buttermilk until just combined.

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

While the cake bakes, make the frosting: melt the remaining butter, stir in cocoa powder, then add powdered sugar and a splash of milk/water until smooth.

When the cake is done, poke shallow holes across the surface and pour the warm frosting over it, spreading evenly.

Allow the cake to set for 10 minutes before slicing and serving.