Preheat the oven to 350-degrees and line the muffin pan muffin liners, or apply cooking spray.
In a large mixing bowl, combine the flour, sugar, salt, baking powder, and cinnamon. Set aside.
Melt the 1/3 cup of butter.
In a small mixing bowl, whisk the egg lightly. Add milk, melted butter, and vanilla.
Add the egg mixture into the bowl with the dry ingredients. Mix lightly and slowly fold in the brown sugar.
Coat the blueberries in flour (or use the streusel mixture to coat) and fold into the batter.
If you're using tall liners, fill the muffin almost to the top of the pan. If you're using standard-sized muffin liners, fill about 3/4 full with batter.
Mix the flour, sugar, salt, brown sugar, cinnamon, and butter together in a bowl. Use a fork to mash the butter into the dry ingredients. Sprinkle about a tablespoon on top of the muffins before going into the oven.
Bake 20-25 minutes. Use the toothpick method to determine if they're done.