Preheat your oven to 350°F (175°C) and prepare a greased 9×13 inch baking dish.

Melt the butter in a saucepan over low heat; let cool slightly.

In a mixing bowl, combine melted butter and sugar; stir until well mixed. Add cocoa powder and mix until smooth.

Gently fold in the sourdough starter until fully incorporated.

Add eggs one at a time, stirring continuously, then mix in the vanilla extract.

Pour the batter into the prepared dish and bake for 25–30 minutes, or until a toothpick inserted comes out with moist crumbs.