4 tablespoons unsalted butter (sliced into 4 pieces and chilled)
1 large egg (at room temperature)
2 tablespoons plus 2 teaspoons sour cream
confectioners sugar for garnish
Preheat oven to 350F. and spray 4 muffin tins with baking spray with flour. (I used cooking spray and lightly sprinkled the tins with flour)
Place granulated sugar, 8 teaspoons of flour, and 4 teaspoons of brown sugar into the bowl of a food processor; process until well combined. Transfer 2 tablespoons of the mixture into a small bowl. To create the streusel whisk in the cinnamon, and 4 teaspoons of brown sugar into the small bowl mixture; set aside.
Add 1/3 cup flour, baking powder, baking soda, and salt to the mixture in the food processor and mix until well combined. Place the butter pieces in the food processor and pulse until small pebbly pieces form. Add the egg and sour cream and mix until well combined.
Place 1 tablespoon of batter into each muffin cup, divide the streusel between the 4 cups and place on top of the batter, and then top each cup with the remaining batter.
Bake for 20 t0 25 minutes or until a toothpick inserted comes out clean.
Let cool for 10 minutes, remove to a wire rack, and let cool for another 10 minutes.