Preheat the oven to 350°F, and prepare a baking try with parchment paper (or use a silicone mat).
In a bowl, add melted butter, egg yolk, vanilla extract and red food colouring. Whisk very well.
Add in white sugar, and continue whisking.
Take the dry ingredients; flour, cocoa powder, and baking soda, and add them into the bowl. Fold gently until just combined into a thick cookie dough.
Divide the cookies into six balls evenly using a spoon or cookie scoop. Roll into granulated sugar and place onto baking mat, with the cookies being evenly spread out.
Bake for 9-10 minutes. The cookies will start to crackle and appear dry, this is when you know they are ready.
Immediately after baking place chocolate hearts into the centre of each cookie. Allow to cool further, or eat warm!
Enjoy!