Clean the damaged area by removing any loose particles of plaster.
Cut a styrofoam piece in approximately the same size and shape of the missing cornice section. Do not to cut a piece that will stick out from the rest of the cornice's plane.
Apply plasterboard adhesive into the gap with a taping knife.
Place the cut-out styrofoam pieces onto the adhesive.
Apply another coat of plasterboard adhesive on top of the styrofoam. Smooth it out with the taping knife. Do not apply adhesive so that it sticks beyond the plane of the cornice. Let it dry for 24 hours.
Sand down any parts of adhesive that stick out over the cornice.
Mix plaster mix according to the manufacturer's directions.
Apply the plaster onto the adhesive with a taping knife, mimicking the rest of the cornice's shape and dimension. Apply another layer of plaster to achieve a more defined cornice shape.
Sand the area after it has dried. Wipe clean with a damp sponge. Paint over the repaired section.