Preheat the oven to 350 degrees. Prep a muffin tin with nonstick spray or muffin liners.
In a bowl, combine one egg, 4 tablespoons of cooled, melted butter, 120 grams of sourdough discard, ¼ cup of milk, ½ cup of brown sugar, and 1 tablespoon of vanilla extract.
Next, add 1 cup of flour, 1 teaspoon of cinnamon, 1 teaspoon of baking soda, and ¼ tsp of salt. Whisk together until incorporated, but don’t overmix.
Next add in the ½ cup of blueberries and fold in gently. If they are frozen, the blueberries will probably turn your batter a purple color.
Portion out the batter into 12 muffin tins. Fill each muffin cup about ⅔ full and bake at 350 degrees for 20 minutes or until a toothpick inserted into the middle comes out clean.
Let cool and serve!