Preheat the oven to 160°C (315°F/Gas 2–3). Grease two baking trays and line them with baking paper.
Using electric beaters, beat the almonds, sugar and egg whites for 5 minutes. Add the flour and vanilla and beat until smooth. Add the zest if using. Drop 1 or 2 level teaspoons of the mixture onto the trays, 3cm (1¼inches) apart, and bake for 10–12 minutes for the small macaroons or 15–18 minutes for the large ones, until firm. Cool on the trays, then transfer to a wire rack to cool.