Wash the raspberries carefully. Clean them and collect in a deep bowl. Sprinkle with sugar and steep for a couple of hours with the lemon juice and the grappa. After this time, pour the pieces of fruit into a medium-sized casserole and cook until syrupy. At this point, whisk everything in a blender, gradually adding the starch and the butter. Pour into the receptacle used earler and thicken over moderate heat, stirring thoroughly. Serve the sauce cold, poured (chilled) over puddings or ice cream.