Preheat your oven to 350°F (175°C). Prepare your muffin tin with colorful cupcake liners.

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

Cream together the softened unsalted butter and granulated sugar using an electric mixer on medium speed until light and fluffy.

Add eggs one at a time, mixing well after each addition. Stir in the vanilla extract until fully incorporated.

Gradually add the flour mixture to the wet ingredients, alternating with sour cream, mixing gently just until combined.

Pour in the whole milk and mix on low speed until the batter is smooth and creamy.

In a small bowl, combine brown sugar with melted butter to create the cinnamon swirl mixture.

Spoon a generous amount of cupcake batter into each lined muffin cup, then add the cinnamon sugar mixture on top.

Top with more batter until filled about two-thirds full, ensuring a delightful swirl of cinnamon sugar.

Bake for 18-22 minutes until a toothpick inserted comes out clean and tops are lightly golden.

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

Prepare the glaze by whisking together powdered sugar, milk, and a splash of vanilla until smooth.

Once cooled, drizzle the glaze generously over each cupcake and let it set before serving.