Rub together the sugar and lemon zest using your fingers to release the oils from the zest.
Add the flour, baking powder, baking soda, cornstarch and salt. Mix until everything is combined.
Add the blueberries and coat them with the flour mixture.
In a different bowl, whisk together the butter, milk, eggs, Greek yogurt, sourdough discard, lemon juice and vanilla bean paste until the mixture is smooth.
Add the wet mixture to the dry mixture and combine everything together. The dough will be on the thicker side, just keep mixing it.
Equally distribute the dough into 12 muffin cups.
For the topping, mix the sugar, flour and butter until it's crumbly. Sprinkle over the muffins.
Bake at 350 for 25-30 minutes, let them slightly cool and enjoy while they're still warm.
