FOR THE STREUSEL:
1/3 cup all-purpose flour
1/3 packed light brown sugar
1 teaspoon ground cinnamon
4 Tablespoons unsalted butter, cold and cubed
FOR THE MUFFINS:
5 Tablespoons unsalted butter, softened
1/3 cup vegetable oil
1 cup sugar
2 large eggs
2 1/4 cups all-purpose flour
2 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
1 cup sour cream
2 teaspoons vanilla extract
1 1/2 cups blueberries, divided
MAKE THE STREUSEL:
In a medium bowl, combine the flour, brown sugar, cinnamon and cubed butter. Using your fingers or two forks, work the butter into the mixture until it is the texture of wet sand. Set the streusel aside.
MAKE THE MUFFINS:
Preheat the oven to 350°F and line a muffin pan with muffin cups.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, vegetable oil and sugar until combined, about 2 minutes. Add the eggs, one at a time, beating between each addition.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Add the flour mixture, sour cream and vanilla extract to the stand mixer and beat until combined. Fold in 1 cup of blueberries.
Layer the batter and streusel mixture into each muffin cup by adding a portion of the batter, topped by a layer of the streusel then another layer of the batter and a final layer of streusel. Top the muffins with the remaining 1/2 cup blueberries then bake the muffins for 18 to 20 minutes or until a toothpick inserted comes out clean. Remove the muffins from the oven and allow them to cool for 5 minutes in the pan before transferring them to a rack to cool completely.