Begin by mixing the 1/2 cup melted butter, eggs, milk, and sugar.
Add the sourdough starter and stir until combined.
Mix flour into the wet ingredients. Cover and let it ferment over night (or up to 48 hours) in the refrigerator. this step is optional, but gives the cobbler a more sour flavor.
The next day, add baking powder and soda to the the batter. Make sure there are no clumps.
For the plum filling, slice plums into a 9x13 inch pan and sprinkle with the flour and spices.
For the streusel topping, press the the sugar, flour, and spices into the butter until it is mixed and creates crumbly clumps.
Spread the batter over the plums and crumble the streusel topping on top.
Bake in a 325° F oven for 50-60 minutes, until golden on top and a toothpick comes out clean.
Enjoy by itself for a delicious breakfast or top with vanilla ice cream for a flavorful fall dessert.