Whisk together the flour, sugar, baking soda, and salt in a large bowl. Make a well in the center of the mixture. Set aside.

Combine the sour cream, cooled butter, egg, and vanilla. Add to the well in the flour mixture. Stir just until combined. (The batter will be thick).

Stir in the chocolate chips.

Divide the batter among the prepared muffin cups, using about a tablespoon of batter per cup. (I use a #50 scoop.)

Bake 10-12 minutes, or until the muffins are golden brown and spring back when touched lightly.

Cool the muffins in the pan for 5 minutes. Then transfer the muffins to a wire rack to cool completely.

Place the melted butter in a small bowl.

In a separate bowl, combine the sugar and cinnamon.

Dip a cooled muffin in the butter, and then roll it in the cinnamon-sugar. Place on a sheet of parchment or wax paper, or on a serving plate. Repeat with the remaining muffins.