Preheat your oven to 350° F.
Butter an 8 x 8” or 9 x 9” baking dish.
In a medium saucepan, melt the butter and the sugar over medium heat. Don't heat for too long, just enough to barely melt the butter. Whisk together until smooth.
Add the the dry ingredients into the butter and sugar mixture and beat until smooth.
Add the vanilla and sourdough starter and whisk again until it's well mixed.
Add eggs, one at a time, beating well after each addition.
The batter should be thin, not like a regular cake batter.
Pour the batter into your buttered baking dish.
Pop the cake into a preheated oven and bake for 25-30 minutes, or until it tests done with a toothpick.
The edges should be pulling away from the sides, but the center will be a little soft.
Put the pan on a wire rack to cool for at least 10 minutes before cutting.