Step 1: Preheat and Prep
Preheat oven to 350°F (175°C). Line a standard muffin tin with paper liners or grease with nonstick spray.

Step 2: Mix the Cinnamon Swirl
In a small bowl, combine brown sugar, cinnamon, and melted butter. Set aside.

Step 3: Cream the Butter and Sugars
In a large mixing bowl, beat softened butter, granulated sugar, and brown sugar until light and fluffy.

Step 4: Add Eggs and Vanilla
Beat in the eggs one at a time, followed by vanilla extract.

Step 5: Add Sour Cream and Milk
Mix in sour cream and milk until smooth.

Step 6: Combine Dry Ingredients
In a separate bowl, whisk together flour, baking soda, baking powder, and salt. Gradually add to the wet mixture, stirring until just combined — do not overmix.

Step 7: Layer and Swirl
Spoon a tablespoon of batter into each muffin cup. Add ½ teaspoon of cinnamon swirl mixture. Repeat with more batter, then swirl gently using a toothpick or skewer.

Step 8: Bake
Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.

Step 9: Cool and Glaze (Optional)
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack. If using glaze, mix ingredients in a bowl and drizzle over cooled muffins.