- With the rack in the middle position, preheat the oven to 190 °C (375 °F).
- In a bowl, combine the dry ingredients. Add the butter and stir until the mixture is just moistened and stick together when pressed between your fingers. Set aside.
- Combine the sugar and cornstarch in a 20-cm (8-inch) square pan with a 2 litre (8 cup) capacity. Add the plums and butter and stir to combine.
- Drop the crumble in pieces onto the plums. Bake for about 45 minutes or until the crumble is golden brown. Serve warm or cold.