Place the orange in a saucepan and cover with water. Bring to the boil, then simmer for 1 hour with a lid on.
Remove the orange from the pan and let cool. Once cool, place in a food processor or blender and blitz until smooth.
Pre-heat the oven to 350F/180C degrees.
Line and grease a baking tin (I use a 8 inch square tin).
In a bowl, place the melted butter, vanilla and eggs and mix well.
Add the orange mixture and blend until smooth.
Add the dry ingredients, one at a time into the orange mixture and blend well.
Pour the mixture into the baking tin, place in the oven and bake for 25 minutes.
Eat and enjoy!