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

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy using an electric mixer.

Add the eggs one at a time, beating well after each addition. Mix in the milk until combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, ground cinnamon, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

In a small bowl, mix the brown sugar and ground cinnamon for the cinnamon swirl. Stir in the melted butter until combined.

Fill each cupcake liner halfway with the cupcake batter. Spoon about 1 teaspoon of the cinnamon swirl mixture on top of the batter in each liner, then add another layer of batter to cover the swirl.

Use a toothpick or a skewer to gently swirl the cinnamon mixture into the batter for a marbled effect.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool completely after removing from the oven.

In a mixing bowl, beat together the softened cream cheese and unsalted butter until smooth. Gradually add the powdered sugar and vanilla extract, mixing until creamy.

Once the cupcakes are completely cool, frost them generously with the cream cheese frosting.