Preheat the oven to 200 °C / 400 °F / Gas 6.
Heat an ovenproof frying pan until very hot, add the duck breasts, fat-side down and cook for 3-4 minutes, or until golden-brown. Turn over and cook for a further 1-2 minutes.
Place into the oven for 8-10 minutes.
Remove from the oven and set the duck aside to rest for five minutes.
Meanwhile, heat a frying pan until hot, add the caster sugar and cook to a caramel.
Remove from the heat then add the orange juice and vinegar, return to the heat and cook for a couple of minutes.
Add the chicken stock, return to a simmer and cook for 3-4 minutes until just thickened.
Add the orange zest then season with salt and freshly ground black pepper.
To serve, carve the duck into thick slices. Finish with a spoonful of orange sauce.