1. Preheat oven to 350 degrees. Line a standard muffin pan with muffin cups.
2. In a large bowl, cream the butter and the sugar. Beat in the eggs, one at a time, until just mixed inches In another bowl, sift together the flour, baking powder, baking soda, salt, and cinnamon. Combine the vanilla, milk and heavy cream.
3. With a spoon, mix a quarter of the dry ingredients into the butter mixture. Then mix in a third of the milk mixture. Continue mixing in the remaining dry and wet ingredients alternately, ending with the dry. Mix until well combined and smooth, but don't overmix.
4. Scoop enough batter into each tin so that the top of the batter is even with the rim of the cup, about 1/2 cup. Bake the muffins until firm to the touch, about 25-30 minutes.
5. To finish, Melt the butter for the dipping mixture in a small bowl. Combine the sugar and cinnamon in another bowl. When the muffins are just cool enough to handle, remove them from the tin, dip them into or brush them all over with the melted butter, and then roll them in the cinnamon sugar.