Beat the butter and sugar for 2 minutes or until pale and creamy. Add the vanilla, egg yolk and melted chocolate and beat until combined.
Add the flour and cocoa powder.
Mix with a spatula or your hands until combined and mixture forms a ball.
Roll tablespoons of mixture into balls and use the back of a rounded teaspoon or your thumb to press in the center of each ball. See note 3
Transfer shaped dough balls to prepared baking sheets, then refrigerate for one hour. see note 4
Bake in a preheated oven of 180°C/350°F (160°C fan forced) for 12 to 14 minutes.
Immediately after baking, re-press indentation into each cookie with the back of a rounded spoon. See note 5
Remove from cookie sheet to cooling rack. Cool completely, about 20 minutes.
Transfer the canned caramel (dulce de leche) into a microwave safe bowl. Heat for 30 seconds then whisk until smooth. Using a spoon or piping bag, divide the caramel between cookies. See note 6