Prepare the Muffins

1. Preheat Oven: Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease the cups with butter.
2. Mix Dry Ingredients: In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and cinnamon until well combined.
3. Combine Wet Ingredients: In another bowl, mix the melted butter, sour cream, milk, eggs, and vanilla extract until smooth.
4. Mix the Batter: Pour the wet ingredients into the dry ingredients and gently fold until just combined. Be careful not to overmix—the batter should be slightly lumpy.

Add the Cinnamon Swirl

1. Prepare Swirl Mixture: In a small bowl, combine brown sugar and cinnamon.
2. Layer the Batter: Spoon half the batter into the prepared muffin cups, filling them halfway. Sprinkle the cinnamon-sugar mixture evenly over the batter.
3. Top with Remaining Batter: Add the remaining batter on top, filling each cup about three-quarters full.

Make the Streusel Topping

1. Combine Ingredients: In a bowl, mix the flour, brown sugar, and cinnamon. Cut in the butter with a fork or your fingers until the mixture resembles coarse crumbs.
2. Top the Muffins: Sprinkle the streusel evenly over the batter in each muffin cup.

Bake

1. Bake Muffins: Place the muffin tin in the preheated oven and bake for 20–25 minutes or until a toothpick inserted into the center comes out clean.
2. Cool: Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.