Melt butter in a frying pan. Add oat flakes and 40 g sugar and roast until golden brown. Rinse and pit fresh plums. Stew lightly in a little sugar water for about 15 minutes. Drain through a sieve, drain well and cool. Crush the plums. Beat egg yolks with 25 g sugar until creamy. Fold the plum puree with the cinnamon into the egg foam. Add the juice of one lemon and whisk repeatedly.
Whip the cream in an ice-cold baking bowl until stiff and gently fold into the plum egg mixture. Sprinkle 4 souffle cups with a tiny bit of oat brittle (keep remaining brittle). Fill the cream into the molds and freeze in the freezer for about 1 hour. Now run the ramekins briefly under hot water and turn them out.
Spread the remaining brittle decoratively and evenly over the parfait.