1Preheat the oven to 375°F.
2Spread the plums out into a pie plate, quiche pan or other similarly-sized baking dish, sprinkle them with sugar and cinnamon and toss them about a bit with your hands, then spread them out again.
3In a bowl (or a food processor) mix the flour, sugar, baking powder, salt and cinnamon; add the butter and blend until crumbly. Sprinkle over the plums, squeezing the mixture as you go to create larger clumps. Bake for 25-30 minutes, until golden and bubbly around the edges.
4Serve warm, with ice cream.