For the Crumble Topping: In a bowl, combine flour, granulated sugar, brown sugar, lemon zest, and salt
Add melted butter and mix until pea-sized crumbles form. Set aside.
For the Muffins: Preheat the oven to 425°F and line a muffin tin with liners
In a bowl, mix together flour, baking powder, and salt. Set aside
In a larger bowl, combine sugar, melted butter, eggs, vanilla extract, and milk. Mix well
Gently fold the dry ingredients into the wet mixture without overmixing
In a separate bowl, coat blueberries with 1 tbsp of flour, then fold them into the batter
Using a scoop, divide the batter evenly among muffin cups. Add toppings, a few blueberries on top, lightly sprinkle turbinado sugar and flaky salt
Top each muffin generously with crumble mixture
Bake at 425°F for 15 minutes, then reduce heat to 375°F (without opening the oven) and bake for about 10 more minutes or until golden
Let muffins cool in the tin for 10 minutes before transferring to a cooling rack
