Combine flour and butter in the large bowl of a food processor. Pulse 5 to 10 times, or until the mixture resembles the texture of wet sand.
Add sugar, egg yolks, orange zest, vanilla, and salt, and pulse several times until the dough forms into a ball on the side of the bowl.
Turn dough out onto a floured surface; form into a flat disc. Cover with plastic wrap and chill at least 30 minutes or up to 3 days.
Preheat oven to 350°F. Line 2 baking sheets with parchment paper. Turn dough out onto a lightly floured surface. Roll dough out, using a rolling pin, to 1/4-inch thickness.
Cut out heart shapes using cookie cutters, re-rolling the scraps as necesary. Transfer cookies to prepared baking sheets as you work. No need to space them out too much, these cookies will not spread.
Bake 20 to 25 minutes or until the tops are no longer glossy. Remove the cookies before they begin to brown around the edges. Immediately transfer cookies to a wire cooling rack to cool completely.
Meanwhile, prepare melting chocolate according to package directions. Stir in food coloring, if desired. Dip cooled cookies in chocolate, or drizzle chocolate over cookies using a piping bag or plastic baggie with the corner trimmed off.