Preheat the oven to 350 degrees F. Spray 20 mini muffin tins with nonstick spray.
PREPARE THE MUFFINS:
In a medium bowl, whisk together the flour, sugar, baking powder and salt. In a separate bowl, whisk together the milk and egg. Slowly whisk in the butter and then vanilla. Add to the dry mixture and stir just until combined. Mix the "swirl" ingredients and gently stir those into the batter (just a little bit... you don't want to mix it totally). Scoop by spoonfuls into the prepared muffin tins.
Bake for about 12 minutes or until the muffins bounce back to the touch and are baked throughout.
ADD THE FROSTING:
In a medium bowl, whisk together the cream cheese and butter (if it's not soft enough, zap it in the microwave for about 10 seconds). Whisk until smooth. Whisk in ½ tablespoon of milk and the vanilla. Add the powdered sugar and whisk that in too. Add more milk, as needed, to create a thick-but-pourable consistency. Remove the muffins from the tins, dip the tops in the frosting, then place them on a wire rack to give the frosting a chance to set. Serve immediately. Store extras in a sealed container at room temperature or in refrigerator.