Create the Crumble Topping
In a medium bowl, combine brown sugar, white sugar, flour, cinnamon, and salt.
Add cold, cubed butter and mix with your fingers until the mixture becomes crumbly.
Refrigerate until ready to use.
Prepare the Batter
In a large bowl, sift together flour, baking powder, baking soda, salt, and poppy seeds. Set aside.
In another mixing bowl, rub sugar and lemon zest together with your fingers until fragrant and pale yellow.
Add sour cream, sourdough discard, melted butter, lemon juice, eggs, and vanilla extract. Mix until smooth.
Gradually fold the wet ingredients into the dry ingredients. Do not overmix.
Gently fold in blueberries using a wooden spoon.
Bake to Perfection
Preheat oven to 425°F (218°C).
Line or grease a muffin tin and divide the batter evenly into 24 muffin cups.
Sprinkle generously with the prepared crumble topping.
Bake for 14-15 minutes, or until golden brown and a toothpick inserted comes out with a few crumbs.
Let the muffins cool in the pan for 10 minutes before transferring them to a wire rack.
Drizzle with Lemon Glaze
Mix lemon juice and powdered sugar until smooth. Adjust consistency with more juice if needed.
Transfer to a small Ziploc bag, snip off a corner, and drizzle over cooled muffins.
