Prep. Preheat oven to 425°F. Grease a half sheet pan and line with parchment paper.
Mix dry ingredients. In a large bowl, whisk together flour, baking powder, baking soda, and salt.
Mix wet ingredients. In another bowl, stir together sourdough starter, milk, egg, sugar, vanilla, and oil until combined.
Combine. Pour wet ingredients over dry ingredients and stir gently until just combined: don't overmix. Lumps are fine.
Pour and top Pour batter onto prepared sheet pan and spread evenly. Sprinkle with desired toppings.
Bake. Bake 18-22 minutes until golden brown and a toothpick inserted in center comes out with a few moist crumbs.
Cool and serve. Cool 5 minutes before slicing into squares. Serve with maple syrup.