Preheat oven to 375 degrees. If using paper muffin cups, place them inside 12-cup muffin pan. If not, generously grease muffin pan cups
In a large bowl, combine dry ingredients. In a separate bowl, combine wet ingredients and whisk well. Pour wet ingredients into dry, and mix until combined and batter is smooth. DO NOT OVERMIX!
Pour about 1/4 cup of batter into each muffin cup. When done, you'll have 1/4 of the mixture left in the bowl
Add brown sugar, cinnamon and melted butter in a bowl, and stir briskly to combine well. It should look like a thick syrup. Add about 1/2 teaspoon of sugar mixture into each cup. Take a toothpick and swirl mixture into batter
Distribute remaining batter equally among all muffins
If the remaining sugar mixture is clumpy, reheat in the microwave for about 10 seconds. Use a spoon to drizzle mixture evenly across all muffins. I like to use a swirl pattern, but it's up to you
Bake for 15-18 minutes, until an inserted toothpick or cake tester comes out clean. Remove muffins from muffin pan, and set on wire rack to cool
In a bowl, add cream cheese, milk, vanilla, and powdered sugar. Whisk well until smooth. Spoon frosting into a piping bag or plastic bag. Carefully cut a very small piece off the end of the bag. Put one hand around the top of the bag, squeezing out the air; carefully hold the bottom of the bag, and drizzle frosting on muffins
Make batter, and store in air-tight container in fridge. I usually do it 1-2 days ahead, at most.
When ready to make, return batter to room temperature before filling muffin cups.
Do not make sugar mixture until ready to bake