Line 8 muffin molds in a tin, or grease with butter. Preheat oven to 375°F. Lightly beat the egg. Put the stewed rhubarb, egg, flour, baking powder, cinnamon, vanilla, oil and sugar in a bowl and gently combine. You want it well-mixed, but ideally don't want to break up all the rhubarb.
Divide the mixture between the muffin molds. Mix together the sugar and cinnamon for the topping and sprinkle over the muffins.
Bake for approximately 15 minutes until golden on top and a skewer put in the middle comes out clean.
If you're concerned about over-mixing the batter or breaking up the rhubarb too much, you can mix together the dry ingredients first then add the rest.
This recipe assumes a slightly tart stewed rhubarb, as in our recipe - if yours is very sweet, you may need a little less sugar.
You can easily scale up this recipe by multiplying up all the ingredients.