Line 2 baking sheets with parchment paper. Set aside.
Using a hand or stand mixer, beat the butter and green food coloring together on medium speed until the butter has turned green, about 1 minute.
½ cup unsalted butter, ¼ teaspoon green gel food coloring
Add the sugar, egg yolk, and vanilla. Mix to combine.
½ cup granulated sugar, 1 large egg yolk, 1 teaspoon pure vanilla extract
Add the flour and baking powder. Mix just until combined, scraping down the sides of the bowl as needed.
1½ cups all-purpose flour, ½ teaspoon baking powder
Roll out the dough to ¼-inch thickness on a lightly floured surface.
Cut out 2-inch circles using a circle cookie cutter or similarly sized glass/cup. Place the cookie dough circles onto the prepared baking sheets (6 cookies per sheet) and freeze for 1 hour.
While the dough freezes, preheat the oven to 350°F.
Remove the cookies from the freezer and bake for 13-15 minutes, or until the edges are golden brown.
Remove the cookies from the oven and immediately place a candy heart into the center of each one. Press the candy heart gently into the cookie until it sticks.
12 candy hearts
Allow cookies to cool on the baking sheet until they are room temperature.