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. Set aside.

In a large mixing bowl, cream the softened unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add the eggs to the butter mixture one at a time, mixing well after each addition, then stir in the vanilla extract.

Alternately add the dry ingredient mixture and sour cream to the large bowl, starting and ending with the dry ingredients.

Pour in the whole milk and blend until the batter is completely smooth.

In a small bowl, mix together the brown sugar and ground cinnamon, then drizzle in the melted butter to create a paste-like consistency.

Spoon the cupcake batter into each liner, filling halfway full. Add a swirl of the cinnamon mixture on top, then cover with more batter.

Bake for 18-22 minutes until a toothpick inserted comes out clean or with a few moist crumbs.

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

Mix powdered sugar, milk, and vanilla extract to make the frosting, adjusting milk quantity for desired consistency.

Once cooled, drizzle the creamy frosting generously over each cupcake.