Cut cold boiled potatoes into small dice; enough to fill 2 cups. Put 2 tablespoonfuls of butter into saucepan, melt slowly, and when it is "bubbling" add 1-1/2 tablespoonfuls of flour mixed with 1 tablespoonful of salt and dash of white pepper. Stir until perfectly blended and smooth. Do not brown. Pour on gradually 1 cup of milk, adding 1/3 at a time - stir and beat to avoid lumps. Cook until smooth and glossy. Then add diced potatoes but do not stir, simply heat and turn into buttered baking dish. Cover with fine crumbs and bits of butter and place them under broiler until crumbs are brown. Serve in same dish.