Cream margarine, sugar and egg. Add warmed milk and stir until smooth. Mix flour with baking powder and cinnamon and stir slowly into the liquid mass.
Core the plums, halve them and cut them into small pieces. Fold into the dough. Pour the dough into the baking tray lined with paper baking cups.
In a small bowl, mix the crumble ingredients to a crumbly dough. Pour the crumble mixture onto the muffins.
Bake at 180 degrees in a preheated oven for about 20-25 minutes.