Line a 12-cavern muffin tin with paper liners, or spray with nonstick spray. Set aside. 

In a medium-size bowl, combine all dry ingredients: flour, sugar, salt, and baking powder. Whisk to combine. Add wet ingredients: melted butter, sour cream, egg, milk, and vanilla. Stir until combined, about 15 seconds. Batter will be thick. Gently fold in blueberries. Using a 4 tablespoon scoop or ¼ measuring cup, spoon batter into prepared muffin tin. 

In a small bowl, combine streusel ingredients and with a fork, mix until crumbly. Spoon about a tablespoon of streusel onto each muffin. 

Bake at 400° for 22-27 minutes, or until top is golden brown and inserted toothpick comes out clean. Do not over bake. {Mine were perfectly done at 25 minutes}.  Let muffins cool in tin for five minutes, then remove to a rack to cool completely. Store in an airtight container at room temperature for up to five days. Make about 12 muffins.  Enjoy!