Place the duck breast into a shallow tray and cover with salt, making sure the underside is completely covered too. Cover the whole tray with plastic wrap and leave at room temperature for 24 hours. Remove the duck from the cure and wipe clean of all salt.
Roll the breast in the black pepper and orange zest and then wrap in muslin, tying with string, making sure the breast is completely covered. Weigh the muslin wrapped duck breast and work out what weight the breast would be once it has lost 35% of its whole weight—this is how much water weight it needs to lose in order to be properly cured—and write it on the muslin somewhere. (Doing it this way means you don’t have to keep unwrapping the duck breast to weigh.) Leave the duck breast in the fridge for about 3 weeks, or until the right weight is reached