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.

In a large mixing bowl, cream the softened unsalted butter and granulated sugar with an electric mixer on medium speed for about 3–4 minutes until light and fluffy.

Next, add the large eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Alternately add the dry ingredients and sour cream to the butter mixture, starting and ending with the dry ingredients.

Gradually incorporate the whole milk into the batter, stirring until you achieve a smooth consistency.

In a small bowl, combine the brown sugar and ground cinnamon, and drizzle in melted butter to create a thick paste.

Fill each cupcake liner about a quarter full with batter, add a spoonful of cinnamon-sugar paste, then top with more batter.

Use a toothpick to gently swirl the cinnamon paste into the batter.

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

Allow the cupcakes to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.

Mix the powdered sugar, additional milk, and a splash of vanilla extract to create a smooth glaze and drizzle over the cooled cupcakes.