Beat together butter and shortening in a large bowl. Add sugars, maple syrup, baking soda, cream of tartar and salt. Add egg yolks and vanilla and beat until well combined. Lastly, beat in flour.
Use a 1 Tablespoon cookie scoop to scoop dough into 1 inch balls and place on baking sheet about 2 inches apart. Bake for 12 to 14 minutes, remove from oven when edges are light golden brown. Frost when completely cool.
Icing: Whisk together whipping cream, melted butter and pure maple syrup. Gradually add powdered sugar and whisk until smooth. Frost cooled cookies and decorate as desired.
Recipe by Crumbs and Chaos at http://www.crumbsandchaos.dreamhosters.com/2014/12/soft-maple-sugar-cookies/