Preheat and prep: Preheat your oven to 350°F (175°C) and line a cookie sheet with parchment paper. This prevents sticking and ensures even baking.
Cream butter and sugar: In a large mixing bowl, beat together 1 1/4 cups sugar and butter until light and fluffy. This step is crucial for a tender, soft cookie.
Add egg yolks: Beat in the egg yolks one at a time, making sure each is fully incorporated before adding the next. This creates a smooth, cohesive dough.
Mix in vanilla: Stir in the vanilla extract until evenly distributed. This really boosts the flavor.
Combine dry ingredients: In a separate bowl, whisk together the flour, baking soda, salt and cream of tartar. Gradually fold them into the butter-sugar mixture. The dough may look dry at first, but keep folding—it will come together beautifully.
Add color: Stir in green gel food coloring until the dough is a uniform, festive green. Adjust the amount for your preferred shade.
Chill dough: Refrigerate the dough for 20 minutes. This step helps the cookies hold their shape and stay soft and thick while baking.
Roll in sugar: Pour the remaining 1/2 cup sugar into a bowl. Scoop 1.5-inch balls of dough and roll them in the sugar to coat completely. This adds sparkle and a slight crunch.
Place on cookie sheet: Arrange the dough balls on your prepared cookie sheet, leaving space between them. Do not flatten—the cookies will spread naturally.
Bake: Bake for 12–13 minutes, until cookies look set but still soft in the center.
Cool: Let the cookies sit on the cookie sheet for 2 minutes before transferring to a wire rack to cool completely. Serve warm or store in an airtight container.