Wash the plums, cut them in half and remove the stones.
Preheat the oven to ~ 350°F (180°C). Grease the muffin tray with a little margarine.
Put the margarine in a bowl together with the date sweetener. If the margarine is still too firm because it has just been taken out of the fridge, soften it in the microwave. Now beat the margarine and the sweetener with a hand mixer until fluffy.
Crack the eggs into a separate container and add them. Beat until well blended!
Then add the rest of the ingredients for the dough (spelt flour, baking powder).
Cut the plums into small pieces and fold into the batter.
Divide the batter among the muffin cups. Leave some room for the crumbles (the muffin cups should be about 3/4 full).
To make the crumble, knead all the ingredients (soft margarine, salt and flour). If you like, add some sweetener here! The dough should be fairly crumbly.
Spread the crumble on the muffins and bake in the oven for 25 minutes.