Preheat oven to 350 degrees
Cover a cookie sheet with parchment paper and set aside
Using a standing mixer, cream together the butter, powder sugar, vanilla and egg and mix until combined and creamy
Using a large bowl, combine the flour, baking soda and cream of tarter
Gradually add in the dry ingredients and mix on low speed
Using your hands continue to mix until fully combined
Lightly flour a cutting board and knead the cookie dough a couple of times
Roll out the dough to about 1/2 inch thick
Cut out a few mittens and place onto the cookie sheet
Bake for 8 minutes or until lightly golden brown
Repeat steps until completely out of dough
Let cookies cool completely
Using the standing mixer, combine the egg whites, cream of tarter, vanilla and powder sugar and mix until combined and stiff peaks form
Split the icing into 4 bowls
Add in a few drops of teal into one bowl and mix until combined
Add in a few drops of red into the second bowl and mix until combined
Add in a few drops of green into the third bowl and mix until combined
Leave the fourth bowl white
Scoop a small amount of frosting into the piping bags
With what is remaining in the icing bags, add in a few tablespoons (about 2) of water into the bowls and mix until combined
Pour the thinned icing into the squeeze bottles and set aside
Using the teal piping bag, pipe an outline around the top half, leaving the bottom bare for the white on the mitten
Fill in with the thinned teal icing
Repeat steps with remaining red and green colors onto the other cookies.
Let cookies dry overnight
Using the white icing piping bag, pipe a outline around the bottom part of the mitten and fill in with the thinned icing
Pour the sprinkles onto a plate
Dip the white part of the mitten into the sprinkles and set aside
Repeat steps on remaining cookies
Let cookies dry for an hour
Using the white icing, pipe a snowflake onto the middle of the mitten
Dip into the sprinkles
Repeat steps with remaining cookies
Choose one of the three colors to pipe a strip across the lower part of the mitten
Let cookies dry for another hour before enjoying.