Pre heat the oven to 180 degrees C / 356 F, I use a fan assisted electric oven, please adjust according to your own oven.
Add the rhubarb,vanilla pod, orange zest, orange juice and castor sugar to a medium saucepan over a medium heat. Cook gently for 10 minutes stirring occasionally. Set aside to cool slightly.
While the rhubarb is cooking make the crumble topping, add the flour, sugar to a large bowl and mix well.
Add the butter then using your finger tips rub the butter into the flour mixture, until you have a rough breadcrumb texture.
In another large bowl add the flour, spices and sugar and mix well. Make a well in the centre then add the eggs, buttermilk and oil. Using a knife mix everything together, when it's all combined add the rhubarb and mix well. You should have a wet better.
Line a 12 hole muffin tray with muffin cases, drop a large dessertspoon full of the batter into each case. Spoon some of the crumble over each muffin.
Bake on the middle shelf for 25 - 30 minutes, check with a metal skewer to make sure they are cooked. They should be golden in colour.
Remove from the oven when cooked, leave them in the tray for a few minutes then transfer to a wire rack to cool down.
Eat warm or cold.