Combine the flour, cocoa powder and salt in a bowl.
In a separate bowl, cream the butter and icing sugar until light and fluffy.
Add the egg and vanilla extract and beat until combined.
Add the dry ingredients in 2 additions, mixing until just combined.
Divide the dough in half and wrap in plastic wrap.
Refrigerate for 1 hour.
Roll the dough out on a floured surface until it is 1/4 inch-thick.
Cut out circles and place on a baking sheet lined with parchment paper.
Use a smaller heart-shaped cookie cutter to cut out a heart in the center of each cookie.
Chill in the freezer for 20 minutes.
Place 2-3 hard candies in the center of each heart.
Bake at 350F for 13 minutes, or until the candies are completely melted and the cookies look fully cooked.
Cool completely on the baking sheet.