Preheat the oven to 350°F and line a 12-cup muffin tin with cupcake liners.
Whisk together the flour, baking powder, baking soda, cinnamon, and salt in a bowl.
Cream the butter and granulated sugar until light and fluffy, about 3–4 minutes.
Add the eggs one at a time, mixing well after each addition.
Stir in the vanilla extract.
Alternate adding the dry ingredients and sour cream, starting and ending with the dry ingredients.
Add the milk and mix gently until the batter is smooth.
In a small bowl, combine the brown sugar and cinnamon, then stir in the melted butter to form a thick paste.
Spoon batter into liners, add cinnamon mixture, and gently swirl with a toothpick.
Top with more batter, filling liners about two-thirds full.
Bake for 18–22 minutes until a toothpick inserted comes out clean.
Cool the cupcakes completely on a wire rack.
Whisk together the glaze ingredients and drizzle over the cooled cupcakes.
