Preheat oven to 400 degrees F and line a muffin tin with cupcake liners.
Start by preparing the streusel topping, In a bowl, mix the brown sugar and 2/3 cups flour. Add the melted butter and mix with a whisk until it turns into a crumbly mixture.
To make the cream cheese filling, simply mix the softened cream cheese, 1/4 cup granulated sugar, and cornstarch until well combined.
In a small bowl, mix the blueberries and 1/2 tbsp of flour until well combined.
To make the muffins, mix 1 1/4 cups granulated sugar and eggs for 2-3 minutes or until it turns a pale yellow color. Add the sour cream, vanilla extract, and vegetable oil. Mix well.
Add 2 cups flour, baking powder, and salt and mix until well combined. Don't over-mix! Fold in the blueberries gently.
Using an ice cream scoop, scoop the muffin batter into each muffin liner, filling each 3/4's of the way.
Top each muffin with 1 tbsp of the cream cheese mixture. Sprinkle the muffins evenly with the streusel topping. Bake for 20 minutes.