Preheat your oven to 400 degrees Fahrenheit. Line your muffin pan with 12 liners.
In a large bowl place 4.5 oz of the sugar, the oil, egg, vanilla extract and the buttermilk. Beat until mixed then stir in the diced rhubarb.
Sift over your dry ingredients into the bowl; wholemeal flour, baking powder, bicarbonate of soda and salt. Stir until just combined and no more so not to make the texture of your muffins too heavy.
Spoon the mixture into your muffin cases, filling them around 3/4 full. Sprinkle their tops with the remaining sugar and bake for 18-20 minutes until golden. A toothpick will come out cleanly when they are done but take care not to over do them.
Leave to stand in the tin for a few minutes before transferring them to a wire rack to cool.