Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a small bowl, combine the brown sugar, 1/3 cup flour, cinnamon, and melted butter to create the streusel; set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the eggs one at a time, followed by the vanilla extract.
Stir in the sour cream and milk until combined.
Gradually add the dry ingredients to the wet mixture, stirring until just combined; do not overmix.
Divide the batter evenly into the prepared muffin liners.
Sprinkle the reserved streusel topping generously over each muffin.
Bake for 20-24 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack.