Preheat the oven to 160ºC. Line a baking sheet with parchment paper.
Finely grate the zest of the lemons. Combine the lemon zest, ground almonds and sugar in a mixing bowl.
In another bowl, beat the egg whites until they form firm peaks.
Gradually fold the egg whites into the almond mixture.
Add the limoncello/lemon juice and stir until combined.
Make about 15 balls from the mixture. If you want them crunchy, roll them in caster sugar, slightly flatten them, then place them onto the baking sheet, allowing enough space for them to expand.
Bake for approx. 15 minutes until the cookies look golden and are cooked through (their base should not be wet).
Let them cool for a couple of minutes in the tray, then move them to a cooling rack.
Dust with icing sugar just before serving.