Preheat the oven to 180C. Lightly grease a 12 hole muffin pan with cooking spray or coconut oil.
In a large mixing bowl, add the rolled oats, milk and vanilla extract. Stir to combine. There shouldn't be too much milk left as the oats will absorb most of it.
Now on top of the oats, add your sourdough starter, egg, coconut oil, shredded coconut, ground cinnamon, baking powder and salt. Stir to combine everything. The mixture should be wet and easy to stir.
Evenly divide the mixture among the 12 muffin holes. Divide the frozen berries among the cups and gently push them down into the oat mixture.
Sprinkle each oatmeal cup with chia seeds and a little brown sugar.
Bake for 40 minutes at 180C (350F). The sourdough oatmeal cups should be easy to remove from the muffin holes once baked.
Cool on a wire rack.