Preheat the Oven: Preheat your oven to 375°F (190°C) and line your muffin tin with paper liners or lightly grease it with cooking spray to keep the bottom of the muffins from sticking.
Mix the Wet Ingredients: In a large mixing bowl, whisk together the melted butter, sourdough discard, eggs, granulated sugar, brown sugar, milk, and vanilla extract. Whisk until the mixture is smooth and well-combined.
Prepare the Dry Ingredients: In a separate medium bowl, whisk together the flour, baking powder, and salt. This will help ensure the dry ingredients are evenly distributed throughout the flour mixture, so your muffins rise properly and have a light texture.
Combine the Wet and Dry Ingredients: Gradually fold the dry ingredients into the wet mixture, stirring gently until just combined. Be careful not to overmix, as this can lead to dense muffins. The batter will be thick and slightly lumpy, which is perfectly normal.
Fold in the Blueberries: Gently fold the blueberries into the batter, being careful not to crush them. If you're using frozen blueberries, there's no need to thaw them—just add them directly to the batter. If desired, you can also add the lemon zest at this stage for a bright, citrusy flavor.
Portion the Batter: Using an ice cream scoop or a spoon, divide the batter evenly among the muffin cups, filling each about two-thirds full. If you like a slightly crunchy topping, you can sprinkle a little granulated sugar over the tops of the muffins before baking.
Bake the Muffins: Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. The tops of the muffins should be golden brown, and the edges should be set.
Cool and Enjoy: Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes. Then transfer them to a wire rack to cool completely. These muffins are delicious warm or at room temperature.