Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, and salt until well mixed.

In a large bowl, cream the softened unsalted butter and granulated sugar together until light and fluffy.

Beat in the eggs one at a time, then mix in the vanilla extract until smooth.

Gradually alternate adding the dry mixture and sour cream to the creamed butter mixture, mixing until just combined.

Gently fold in the whole milk to your batter.

In a small bowl, combine the brown sugar and ground cinnamon with melted butter to form a thick paste.

Fill cupcake liners about one-third full with batter, add a spoonful of cinnamon mixture, and swirl gently.

Bake for 18-22 minutes, or until a toothpick inserted comes out clean.

Allow the cupcakes to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.

Whisk together the powdered sugar, milk, and vanilla extract until smooth to make the glaze.

Drizzle the glaze over the cooled cupcakes before serving.