1.Preheat the oven to moderately hot, 200°C (180°C fan-forced). Grease a 12-hole ( 1/3-cup/80ml) muffin pan.
2.Place flour and caster sugar in large bowl; and the combined butter, buttermilk, egg, vanilla and 1 cup (110g) of the rhubarb; mix until just combined.
3.Divide mixture among holes of prepared pan; sprinkle with remaining rhubarb and combined demerara sugar and cinnamon.
4.Bake for about 20 minutes. Cool on wire rack.