Preheat the oven to 180C/350F/Gas 4, and line a baking tray with parchment paper, or a silicone mat.

Using a handheld or stand mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy. Add the egg yolk, and vanilla, and mix until combined.

Add the melted chocolate, and mix until combined.

Add the flour, and salt, and mix to form a smooth, slightly sticky dough.

Roll the dough into tablespoon-sized balls (about 0.7oz each). Roll each one in granulated sugar, and place on the prepared baking tray.

Use your thumb, or the back of a measuring spoon (I used a 1/2 teaspoon measuring spoon), to make an indent in the middle of each ball.

Bake for 10 - 12 minutes until slightly firm, and cracked around the edges. If the indents have risen, gently press them back down with the measuring spoon.

Allow to cool for 5 minutes before transferring them to a wire rack to cool completely.

Fill the indents in the cookies with the chocolate caramel sauce. Place the cookies in the fridge to allow the filling to set.

To speed up chilling time, place them in the freezer for 30 minutes. If desired, drizzle on some extra melted chocolate.