Preheat oven to 425℉ and line 12 muffin cups with baking cups or grease them with a little butter.
Make the streusel.
Add flour, sugar, and salt to a medium bowl and whisk to combine. Melt the butter, let cool, then pour over the dry ingredients. Mix until moist but still crumbly, like wet sand. Set aside.
Next, make the muffin batter.
Sift together flour, salt, baking powder, and sugar.
In a separate bowl, whisk together the eggs, milk, oil, and vanilla. Add this to the dry ingredients and gently stir until just combined. The mixture should be fully moist, but still have a few lumps in it. The trick to getting light, fluffy muffins is not over mixing them.
Stir blueberries into the batter.
Fill muffin cups with batter. Using your fingers, crumble the streusel topping over each one.
Bake on center rack for 20-25 mins, or until a toothpick stuck into the center of a muffin comes out clean and the streusel topping is starting to brown.