Lightly toast the almonds in a dry frying pan on the stove.

Transfer the almonds to a food processor and grind them to a powder by pulsing on and off to keep the powder loose.

Mix the almonds with the powdered sugar and flour. Set aside.

In a separate a bowl, beat the egg whites to soft peaks. Gradually beat in the granulated sugar until the mixture forms stiff peaks. Gently fold in the lemon zest and almond extract.

Pipe the mixture with a pastry bag using a round 1/4-inch tip into 1 1/2-inch rounds onto baking sheets lined with lightly buttered parchment paper.

Bake in a preheated 275°F oven for 1 hour. Check them periodically to make sure the Amaretti biscuits don't brown.

Turn off the oven and let the Amaretti dry out in the oven for an additional hour. They should be very crisp.

Store them in an airtight container, dated and labelled.