Make the crumb topping first. Add the melted butter, sugar, and flour to a small bowl and work it together with a fork until it forms coarse, chunky crumbles. Stop while the pieces are still large. Set aside.

Heat the oven to 350°F (176°C). Line a 12-count muffin tin with paper liners, or grease each well generously so the muffins release cleanly.

Combine the dry ingredients. In a large bowl, stir together the flour, sugar, baking powder, baking soda, cornstarch, lemon zest, and salt with a fork. Tip in the blueberries and toss until each berry is dusted with the flour mixture. Set aside.

Whisk the wet ingredients. In a medium bowl, whisk the melted butter, eggs, sourdough discard, sour cream, and vanilla until smooth and uniform.

Bring the two together. Pour the wet mixture over the dry and fold with a spatula only until no dry flour remains. A few lumps are fine and expected. If the batter is too stiff to spoon easily, loosen it with milk or water, one tablespoon at a time.

Fill and top. Divide the batter evenly among the 12 wells. Scatter the crumb topping over each muffin and press it gently into the surface so it stays put during baking.

Bake for 30 to 35 minutes, until the tops are set and a toothpick inserted into the center of a muffin comes out clean. Cool in the tin for 10 minutes, then transfer the muffins to a wire rack to finish cooling.