Preheat oven to 375℉.
Using a KitchenAid stand mixer or hand mixer, first mix together flour, sugar, salt, and baking powder.
Then, add butter, lemon zest, eggs, milk, and vanilla extract and mix just until all ingredients are incorporated together.
Finally, by hand, gently fold blueberries into batter.
Spoon batter into a well greased or lined muffin pan, filling each cup about two-thirds.
Bake for 20-25 minutes. The muffins will start to rise by this time.
In the meantime, make the streusel topping. In a bowl, mix together melted butter, flour, brown sugar, and a pinch of salt. Toss together using a fork until small crumbs form.
Remove muffins from oven, gently brush the top of each muffin with butter, and sprinkle with streusel topping.
Bake another 20 minutes or until a toothpick inserted in the center comes out clean. Enjoy!