Beat butter and sugar in mixing bowl for 4-5 minutes or until light and fluffy. Beat in egg yolks and vanilla extract until just combined.
Sift together the flour, cocoa powder and salt in bowl. Add the flour mixture to the wet ingredients and mix until just combined on low speed.
Add powdered sugar to rolling pin and parchment paper. Roll dough to ¼" thickness and place in fridge for 1 hour.
Cut into desired shapes and place on parchment-lined baking sheet. Bake at 350°F for 10-12 minutes or until the cookies are firm on the edge.
In a large bowl using a hand-held mixer, beat all of the icing ingredients together on high speed for 5 minutes until stiff peaks form.
Decorate the sugar cookies with icing and let set for an hour for icing to harden.
1. Use dark cocoa powder to give a more intense chocolate flavor. 2. Use parchment paper and add powdered sugar to rolling pin so cookie dough doesn't stick when rolling. 3. Roll out chocolate sugar cookie dough first before placing in fridge for 1 hour so it is easier to get to ¼" thickness.