Overnight Sourdough Buckwheat Pancakes

Stir all-purpose flour, buckwheat flour, sourdough starter, and milk together in a 1-quart container with a tight-fitting lid until evenly combined. Cover with lid and let sit on the counter, 8 hours to overnight.

Stir in eggs, butter, baking powder, vanilla extract, and salt the next morning. Whisk until smooth.

Spray a skillet with cooking spray and heat over medium heat. Pour about 2 tablespoons of batter into the skillet for each pancake and cook until bubbles form and the tops look dry, about 2 minutes. Flip pancakes over and cook for 1 more minute.