Using a stand mixer, cream butter, powdered sugar, vanilla, and salt until smooth.
Mixing on low speed, slowly, add flour until combined. Add food coloring and mix to make the dough green.
Remove dough from mixer and place on parchment paper. Form into a 12- to 14-inch log. It is easier to get a uniform log is you start the dough at one side of the paper, form, and then roll it into a log. freeze at least 30 minutes or until firm. Note: If you freeze the dough for longer than 30 minutes let it sit for 20-30 minutes so you are able to cut it and get even slices. If your slices are not even rounds the cookies will not cook evenly.
When ready to bake the cookies, preheat oven to 350°F. Cut the shortbread dough into ⅓ -½-inch slices and bake for 12-13 minutes on parchment lined or slip-mat lined baking sheets.
Remove cookies from oven and immediately place one heart sprinkle on each cookie while they are soft. Cool on a cooling rack.