Preheat your oven to 375°F.  Line a 12-cup muffin tin with paper liners or grease it with butter to prevent sticking.

Make the Streusel Topping:
In a bowl, mix together granulated sugar, brown sugar, flour, cinnamon, and salt. Cut in the cold butter using a fork or pastry cutter until the mixture resembles coarse crumbs. Refrigerate until ready to use.

In a bowl, mix the melted butter and granulated sugar until well combined. Add eggs one at a time, beating well after each addition. Stir in the milk and vanilla extract.

Add the flour, baking powder, and salt. Next gradually stir everything together until just combined. Be careful not to overmix! Fold in the blueberries.

Divide the batter evenly among the muffin cups, filling each to the top. Sprinkle generously with the prepared streusel topping.

Bake for 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.