Preheat the oven to 180°C/350°F/gas 4.

Line two trays with greaseproof paper and rub with olive oil.

Cube the butter and place in a food processor with the polenta, flour and sugar.

Finely grate in the zest of 1 lemon (or orange), then pulse to combine.

Squeeze in the juice of half a lemon (or orange), and pulse again to bring the mixture together into a ball of dough.

Divide into 24 pieces, roll into balls and place on the trays, leaving a 5cm gap between them.

With your thumb, create a 1cm-deep dent in the centre of each ball.

Finely grate the remaining lemon (or orange) zest and scatter into the dents, followed by a little sprinkle of caster sugar.

Bake for 10 minutes, or until lightly golden.

Transfer to a wire rack to cool, then tuck in.