Whisk egg whites until soft peaks have formed. Add the caster sugar a little at a time and keep whisking until you have a shiny and stiff mix.
Fold in the ground almonds, icing sugar & flour just enough to combine ingredients but without overworking it.
Scoop the mix into a piping bag and pipe 1-1.5" mounds onto a baking tray.
Dip a finger into a glass of water and carefully tap the top of each mound to smooth it over.
Bake for 35-40 minutes until a light golden brown.
Switch the oven off but leave the amaretti in the oven with door slightly open until they are completely cool. This will keep them crisp and allow any moisture to evaporate.