In a medium sized bowl whisk together butter and sugars until well combined. Add the egg and vanilla and mix well.
Sift in the flour, baking soda, salt, and cinnamon and then fold until well combined.
Cover with plastic wrap and chill dough for 30 minutes. Once chilled scoop into 1 oz balls onto a lined baking sheet and bake at 350 degrees for 12 minutes.* Remove cookies to a wire rack to cool completely before frosting.
While the cookies are cooling mix together all the ingredients for the frosting.**
Once cookies are cooled dip the top of each one into the frosting and allow them to set for at least 20 minutes. Cookies will be stackable after an hour or two.
*Be careful not to overbake. 12 minutes is perfect every time I make these!
**You can add more milk if your mixture is too thick but be careful not to add too much. You want your frosting to be thin enough to dip but thick enough that it won't run everywhere.