Preheat the oven to 350°F. Line two large, rimmed baking sheets with parchment paper and set aside.
Cream butter and granulated sugar until lightly and fluffy, 3-4 minutes using an electric mixer. Scrape down the sides at the half-way point.
Add the egg and vanilla and mix again for an additional minute. The batter will become lighter in color.
In separate mixing bowl, whisk together the flour, baking powder, baking soda and salt.Add the dry ingredients to the wet ingredients and mix until combined.
Scoop the cookie dough into rounded balls about 3 tablespoon in size. Place cookies on the baking sheets a couple inches apart (about 8 cookies per baking sheet).
Bake for 12-13 minutes until slightly crackly on top.Let the cookies cool for a few minutes on the baking sheet before removing to a cooling rack.
For Icing
Using an electric mixer, whisk egg whites for 1 minute till foamy.
Add powdered sugar, cream of tartar and salt to egg whites and continue to whisk. Mix until the egg whites are extremely light and fluffy, almost like whipped a fluffy whipped cream.
Add water by the tablespoonful until it has the consistency of hot fudge. Typically this takes between 3-5 tablespoon of water to get the right consistency.
Icing dries out somewhat quickly, so keep covered until ready to use
For Decoration
Using the black icing, pipe swayed lines to mark the string of lights. Place mini m&m's on to the string. Icing will harder as you do this so paint a few string at a time versus the entire batch.
Notes
Cookie Recipe adapted from Mel's Kitchen CafeIcing Recipe adapted from New York Times