Preheat oven to 350º.
In a medium bowl, whisk together flour, cream of tartar, baking soda and salt; set aside.
In a large bowl using an electric mixer, cream together butter and sugar until light and fluffy. Beat in eggs, one at a time, then beat in lemon zest and lemon emulsion if using. Add flour mixture; mix at low speed until well combined.
In a small bowl, mix together crushed lemonheads and yellow sugar.
Roll dough into 1″ balls, then roll in sugar mixture. Place 2″ apart on parchment-lined cookie sheets and bake for 10 - 12 minutes until cookie is set and bottoms start to brown. Remove to a wire cooling rack.