Preheat your oven to 350°F (176°C). Line a 12-cup muffin pan with parchment muffin liners or grease each cup lightly with oil or butter. This will prevent sticking and make cleanup easier.

Prepare the crumb topping first so it’s ready when you need it. In a small bowl, combine the melted butter, sugar, and flour. Stir with a fork until the mixture becomes crumbly. Set aside.

Whisk the dry ingredients in a large mixing bowl: flour, sugar, baking powder, baking soda, salt, cornstarch, and lemon zest. Stir until well combined.

Toss the blueberries gently into the dry mix. Coating them in flour prevents them from sinking to the bottom of the muffins during baking.

Mix the wet ingredients in a separate bowl. Whisk together the melted butter, eggs, sourdough discard, sour cream, and vanilla extract until smooth and well blended.

Combine the wet and dry ingredients. Pour the wet mixture into the dry bowl. Stir gently using a spatula or wooden spoon just until everything is incorporated. Avoid overmixing—it’s okay if there are a few streaks of flour. If the batter feels too thick, add a tablespoon or two of milk or water to loosen it.

Spoon the batter evenly into the muffin cups, filling each one about three-quarters full.

Sprinkle the crumb topping generously over each muffin, gently pressing it down with your fingertips so it sticks to the batter.

Bake in the center of the oven for 30 to 35 minutes, or until the tops are golden and a toothpick inserted in the center comes out clean.

Cool the muffins in the pan for about 10 minutes, then carefully transfer them to a wire rack to cool completely. This prevents soggy bottoms and helps preserve that perfect muffin texture.