In a bowl, whisk the soft butter with the sugar. Add the lemon zest, vanilla extract, salt and milk and keep whisking until well combined. Add the flour and baking powder and whisk until the batter is smooth. Add the blueberries, mix and pour the batter into a 12ct nonstick muffin pan.
Preheat the oven to 355 °F (180 °C).
Form the crumble by mixing all the ingredients with your hands, then sprinkle it on top of each muffin. Bake for about 25 minutes, or until you stick a wooden pick in a muffin and it comes out dry with just a few moist crumbs.