Preheat oven to 375°F (190°C). Line a muffin tin with paper liners.

Make the cinnamon filling: In a medium bowl, cream together the softened butter and brown sugar until light and fluffy. Stir in the cinnamon.

Make the muffin batter: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.

Fill each muffin cup about halfway full with batter. Dollop 1-2 tablespoons of cinnamon filling on top of the batter in each muffin cup.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.

Make the frosting: Beat together the softened cream cheese and butter until smooth. Gradually add the powdered sugar, beating until light and fluffy. Stir in the vanilla and milk until desired consistency is reached. Frost the cooled muffins and enjoy!