Preheat oven to 220°C (200°C fan forced).

To make caramel, pour condensed milk into a ceramic pie dish or shallow ovenproof dish, cover tightly with foil. Place dish in medium baking dish, add enough boiling water to come halfway up the side of pie dish. Bake about 1½ hours or until milk is caramel in colour. (Add extra boiling water to dish as needed to maintain water level.) Remove pie dish from baking dish, cool. Whisk caramel until smooth, refrigerate until ready to use.

Have butter at room temperature.

Reduce oven temperature to 160°C (140°C fan forced). Line oven trays with baking paper.

Beat butter, rind and sifted icing sugar in small bowl with electric mixer until light and fluffy. Stir in sifted flour, cornflour and cinnamon, in two batches.

Dust hands with flour, roll rounded teaspoons of mixture into balls, place about 2.5cm (1-inch) apart on trays. Dip a fork into a little extra flour; flatten biscuits gently with fork.

Bake shortbreads about 10 minutes. Stand shortbreads on trays 5 minutes, before turning onto wire racks to cool.

Sandwich short-breads with caramel. Dust with extra sifted icing sugar before serving.