Cream the 2/3 cup unsalted butter, softened with the 1 cup granulated sugar in a large mixing bowl.
Add in the beaten 2 large eggs, beaten, followed by the 1/2 teaspoon vanilla extract.
In a medium sized mixing bowl, sift or whisk the 3 cups all-purpose flour, 1/2 teaspoon salt, 2 1/2 teaspoons baking powder, 1/2 teaspoon baking soda and 1/2 teaspoon ground nutmeg.
Add the dry ingredients and 1 cup sour cream alternately to the butter mixture in two separate additions, beginning and ending with the dry ingredients. Mix only until just combined.
Heavily grease a muffin tin. Scoop muffin dough evenly into each tin, filling about 2/3 full. Bake muffins at 350°F for 18-20 minutes, or a toothpick inserted in the center of the muffin comes out clean.
Once muffins are removed from oven, allow to sit 1-2 minutes and then run a knife around the edge of each muffin. Allow to cool in the muffin tins about 5 minutes, and then remove and place on a wire rack to cool.
Melt 1/3 cup unsalted butter in a small bowl. In a separate bowl, combine the 1/2 cup granulated sugar with the 1 teaspoon ground cinnamon.
Dip the top of each muffin first in the melted butter, then in the cinnamon-sugar mixture. May dip tops of muffins while they are still warm.