Prepare muffin cases by cutting out 8 squares of baking paper, or use paper cases to line 8 x 125ml (1/2 cup) muffin pans.
Mix the sifted flour, cinnamon and caster sugar together in a bowl.
In a separate bowl, whisk together the melted butter, egg, yoghurt and milk/butter milk. Add the dry mix to the wet mix and combine to form a smooth batter. Fold in the slices of rhubarb.
Spoon the mix into the prepared muffin tins and sprinkle each muffin with oats and raw sugar.
Bake for 20–25 minutes, or until risen and golden in colour. Allow to cool, and then dust with icing sugar to serve.
*The activity of the three active cultures is depleted by cooking and heating