Place orange in a deep saucepan, cover with water. Place saucepan over high heat & bring to a boil. Place a lid on it & reduce heat to low. Simmer until the orange is very tender when pierced with a sharp knife. Drain, quarter & set aside to cool completely.
Preheat oven to 350 F. Lightly butter six-1 cup ovenproof baking dishes.
Place rhubarb, brown sugar, spices, vanilla & water in a heavy based saucepan over medium heat. Bring to a simmer & cook, stirring occasionally, for 5-6 minutes until rhubarb thickens. Remove from heat & set aside.
Place cooled orange quarters with the skin on into a food processor & puree until smooth. Add flour, butter, buttermilk, sugar & eggs & process until smooth.
Divide batter among prepared baking dishes. Place on a baking tray & bake for 40 minutes or until tops are golden.
Serve warm topped with spiced rhubarb & whip cream.