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

Add the dark chocolate to a microwave-safe bowl, and heat in 30 second intervals, stirring after each one, until melted and smooth. Set aside.

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

Add the flour, cocoa powder, 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.

Add the white chocolate, and heavy cream to a microwave-safe bowl, and heat for 20 - 30 seconds. Stir until melted and smooth. Once melted, stir in the mint extract, and a drop of green food colouring.

Fill the indents in the cookies with the mint chocolate filling. Place the cookies in the fridge for about 30 minutes to allow the filling to set. If desired, drizzle on some extra melted chocolate.