Preheat oven to 350°F. Line two cookie sheets with silicone baking mats or parchment paper.
Cream butter and sugar in the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a hand mixer). Cream until the mixture is fluffy, about one minute.
Add the egg, egg yolk, and vanilla and mix until smooth, then mix in baking soda, cream of tartar, and salt. Add flour and mix until cookie dough forms.
Add a few drops of green food coloring and stir to mix throughout. How much food coloring depends on the kind (gel vs. grocery store food coloring). Add more as needed to get the green color you want.
Scoop 2 tablespoon sized balls 2-inches apart on cookie sheet. Place one heart sprinkle on one side of each cookie. Bake for 11-15 minutes, or until the bottoms are just starting get golden and the top is no longer glossy. Cool at least 10 minutes on cookie sheet before removing.
Alternately, if you want to know exactly where your sprinkle will land, press the sprinkle on the cookie as soon as it comes out of the oven.
Store in an airtight container for up to 3 days or freeze for up to a month.