Line 6 muffin molds in a tin, or grease with butter. Preheat oven to 375°F/190C.
Lightly beat the egg. Add the melted butter, yogurt and vanilla, whisk to combine.
Separately, mix the flour, sugars, baking powder and cinnamon.
Add the sliced rhubarb to the flour mixture along with the wet ingredients. Mix so that everything is combined and well-distributed but try not to overmix. If it is very stiff as you mix, add a little more yogurt but it should be on the firmer side.
Divide the mixture between the muffin moulds and smooth off the tops a little.
Mix together the sugar and cinnamon for the topping and sprinkle over the muffins.
Bake for approximately 18-22 minutes until golden on top and a skewer put in the middle comes out clean.