Wash, clean, halve and stone the plums. Mix with the cinnamon sugar and place rather flat in a greased casserole dish or ovenproof portion bowl.
For the crumble mix the dry ingredients, use the butter to make crumble (works well with the electric mixer). Spread the crumble on the plums and place in the oven for about 30 minutes at 170 ° C on the middle rack.
Serve the crisp warm, with a scoop of vanilla ice cream if you like.