In a large bowl, mix together the flour, milk, sugar, sourdough starter, eggs, butter and salt. Cover the bowl with a plate or a lid and pop it into the refrigerator overnight (or up to 24 hours.)

In the morning, whisk in the baking powder and baking soda.

Preheat a cast-iron or non-stick pan over medium-high heat, then grease the pan with butter.

Ensure the pan is hot, and the butter is sizzling, then pour in around ⅓ cup of batter for each pancake. Drop a few blueberries into the batter in the pan, about 5-6 per pancake. If the pan gets too hot, turn down the heat a little once you've poured in the batter. 

The batter will spread out in the pan, and won’t rise just yet. Bubbles will start appearing on the surface of the pancake. Once bubbles cover the whole area (about 2-3 minutes), flip the pancake over and finish cooking the other side for another minute or 2. 

Transfer the pancake to a plate and continue with the rest of the batter until it is all used up, stacking the pancakes in a pile as you go. 

Serve with butter and maple syrup.