Step 1: Preheat and Prepare

Preheat oven to 375°F (190°C).

Line a 12-cup muffin pan with liners or grease lightly.

Step 2: Make the Streusel Topping

Mix brown sugar, flour, and cinnamon in a bowl.

Add melted butter and stir until crumbly.

Refrigerate while preparing the batter.

Step 3: Prepare the Muffin Batter

In a medium bowl, whisk flour, baking powder, baking soda, and salt.

In another bowl, beat butter and sugar until creamy (2–3 minutes).

Add eggs one at a time, then vanilla.

Alternate adding dry ingredients and sour cream, mixing gently until combined.

Step 4: Create the Cinnamon Swirl

Combine brown sugar and cinnamon in a small bowl.

Step 5: Assemble the Muffins

Fill each muffin cup with 1 tablespoon of batter.

Sprinkle ½ teaspoon of the cinnamon-sugar mixture over it.

Add another layer of batter, filling cups two-thirds full.

Top with streusel mixture.

Step 6: Bake

Bake for 18–22 minutes until golden and a toothpick inserted comes out clean.

Cool in pan for 5 minutes, then transfer to a rack.

Step 7: Add the Glaze (optional)

Whisk powdered sugar, milk, and vanilla.

Drizzle over cooled muffins for a bakery-style finish.