Instructions: Preheat oven to 375ºF. Place muffin liners in a 12-hole muffin tin.
In a large bowl, combine 2 cups of flour, baking soda, baking powder and salt; set aside.
Using an electric mixer, beat egg with 3/4 cup of sugar in a large bowl until light and fluffy. Add sour cream and beat until thoroughly combined; blend in vanilla extract.
Add half of flour mixture and half of milk to sugar mixture; beat until just combined. Add remaining flour mixture and milk; beat until smooth.
Fold in blueberries and fill each muffin liner about 3/4 full; set aside.
To make streusel topping, in a small bowl, combine remaining 1/4 cup each of sugar and flour. Pour in melted butter and combine with fingertips. Divide crumb mixture over muffins; gently press into top of muffin batter with fingertips.
Bake until slightly golden and a toothpick inserted in center of a muffin comes out clean, about 30 to 35 minutes. Cool in pans for about 10 to 15 minutes and then transfer muffins onto racks to cool completely. Yields 1 muffin per serving.