Preheat the oven to 170C/325F. Line two baking trays with a silicone mat or baking parchment.

Using a hand whisk or mixer, whisk the egg whites to stiff peaks. Fold in the ground almonds, caster sugar and vanilla extract. Slowly stir in the amaretto. Add just enough so that the mixture is a thick paste that can be formed into a ball in your hand (it should be very sticky though).

Place the caster sugar and confectioners sugarr for rolling on two separate plates. Using a teaspoon, scoop out a chunk of the cookie mixture and roll it into a ball in your hands (about the size of a walnut - note, it will be sticky). Roll it in the caster sugar, then roll it in the icing sugar and place on the baking tray.

Repeat with the remaining dough until you've used it all up. Leave a bit of space between the cookie balls as they will expand slightly during cooking.

Use your finger to squash the cookie balls just a tiny bit, then place in the oven to cook for 15 minutes.

Take out of the oven, leave to cool for 5 minutes then remove from the baking tray. You can serve them warm, or allow them to cool completely on a cooling rack.