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 icing sugar 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 a little (they will crack a little at the edges - that's fine) 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.
**Note 1 ** Ground Almonds:
It's best to use store-bought ground almonds (if you're in the US, use blanched almond flour - the fine grainy kind (NOT the powder kind that's like regular flour)) - for this recipe. Please don't try using home-ground almonds, as grinding them yourself can result in ground almonds that are a little too fine, and slightly oily for the recipe - this can cause the amaretti to go flat.I've tested this recipe with both regular UK ground almonds, and with blanched almond flour that I picked up in the US last time I was there. When testing the US almond flour, I used Full Circle Market Almond flour, and this produced the same results as the regular UK ground almonds.
Storing the cookies:
These cookies can be stored in a sealed jar at room temperature. They should last for at least 1 week (up to two)..Nutritional information is per cookie.