These sourdough pancakes are made with an overnight batter and kefir!
In a large mixing bowl, whisk together sourdough starter, flour, and kefir. Cover and allow to ferment overnight, or about 8-12 hours (see note).
Whisk in the butter, eggs, maple syrup, and salt. Sprinkle the baking soda on top of the batter and whisk just to combine well. The batter will be quite thick, but you can thin it out with extra kefir if needed.
Preheat a cast iron skillet or griddle over medium heat. Add a good amount of butter. Ladle the batter onto the hot skillet. Cook for about 2 minutes, or until the bottoms are nicely browned and the pancakes are bubbling in the center. Flip and cook for an additional 2 minutes, or until nicely browned on the bottom as well.
Repeat with remaining batter, adding more butter with each batch.
Serve hot with lots of butter and a drizzle of maple syrup.