Pre-heat the oven to 170ºC/150ºC fan.
Line your baking tray with baking parchment or a non-stick baking mat (I love these bake-o-glide sheets).
Roughly chop the dark chocolate into 16 equal(ish) sized pieces - they don't need to be perfect as they're hidden inside the biscuits.
Put your egg whites (2 large or 70g) into a large bowl and beat on a medium speed until they form a firm peak.
Put the ground almonds (200g) and caster sugar (150g) into a large bowl. Fold in the beaten egg whites and then add the Amaretto liqueur (1 tbsp) and vanilla extract (¼ tsp). Stir until you have a smooth paste.
Put your remaining caster sugar (3 tbsp) onto a large plate and the icing sugar and cocoa powder (1½ tbsp of each) together on a separate large plate.
Divide the biscuit mixture into 16 equal sized ball. Take two-thirds of a ball and roll it into a smaller ball. Make an indent with your finger in the middle and nestle a piece of dark chocolate in the indent. Roll the remaining third into a disk and place it on top of the chocolate. Pinch around the edges to seal the chocolate inside.
Roll the biscuit in the caster sugar and then in the icing sugar/cocoa powder mix and place it on the baking tray. Flatten it a little (if you want, this isn't essential it's just how I like them to look).
Repeat until you've made all 16 biscuits, then place the tray in the oven for 15-17 minutes until they look just like the ones in the pictures.
Once they're cooked remove them from the oven and leave them to cool for a few minutes on the tray before transferring them to a wire rack to cool completely.