I like to eat this when the plums are warm, and I sometimes have it with a slice of pandoro or panettone. If the season is right, you can use peaches or nectarines instead of the plums. Save any leftover amaretti crumbs to sprinkle over ice cream.
Preheat your oven to 200°C (400°F/Gas 6). Crush the amaretti biscuits to coarse crumbs. Cut through the plums all the way around through to the stone. Hold the plum in your palm and twist the other half away so that it comes off smoothly. Sit the plums cut side up in a baking dish. Rem