Get Ready! Preheat your oven to 350°F (175°C). Line a small baking sheet with parchment paper or a silicone mat for easy cleanup.
Mix the Wet Goodness: In a medium bowl, gently whisk together the melted butter, granulated sugar, egg yolk, vanilla extract, and the vibrant red gel food coloring until beautifully combined and smooth.
Fold in the Dry: Sprinkle the all-purpose flour, unsweetened cocoa powder, baking soda, and a pinch of salt evenly over your red mixture. Stir just until everything comes together into a soft, lovely dough. Try not to overmix!
Shape Your Love: Gently press the dough flat in the bowl, then divide it into 6 equal portions. Roll each portion into a smooth ball in your hands, then lightly roll it in a bit of extra granulated sugar for a lovely sparkle. Place them evenly spaced on your prepared baking sheet.
Bake to Perfection: Pop them into the preheated oven for 10-12 minutes. You'll know they're ready when they've spread slightly, have delicate crackles on top, and appear set and dry.
Cool & Add a Heart: Let the cookies rest on the baking sheet for just 2 minutes before carefully transferring them to a wire rack to cool. Once they've cooled for about 10 minutes (they'll still be a little warm), gently press one of your heart-shaped chocolates into the center of each cookie.
Chill for a Bit: For the best results and to ensure those chocolate hearts set perfectly, move the cookies to the fridge for about 30 minutes. Let them come to room temperature before sharing and enjoying your delightful, heartfelt treat!