Preheat the oven to 180°C (approximately 350°F). Fill a muffin tin with paper baking cups.
Rinse and finely chop the rhubarb. Rinse the orange in hot water, then pat it dry and grate the peel. Squeeze out the juice. Combine the egg, grated peel, juice, vegetable oil, sugar, vanilla sugar, salt and buttermilk. Mix the flour and baking powder together in a mixing bowl. Stir the buttermilk mixture quickly into the flour mixture until moist. Fold in the rhubarb and fill the baking cups with the batter. Bake on the middle rack until golden brown, or for about 25 minutes. Remove the muffins from the baking sheet and let cool on a wire rack.