Cut the lobster in two lengthwise, remove the meat carefully, as the half-shells must be kept whole, and chop the meat finely. Melt 1½ oz. butter and pour it on the lobster. Add the white breadcrumbs and the sauce, season rather highly with cayenne and mix well. Press the mixture lightly into the shells, cover with brown breadcrumbs, put 3 or 4 pieces of butter on top and bake for about 20 min. in a moderate oven (350° F., Gas 4). Serve hot or cold.