Preheat oven to 160 deg C. Grease a 20cm diameter cake tin and line the base with baking paper.
Cut the orange into quarters, remove the seeds and whizz in a food processor until finely chopped.
Dissolve B soda in 1/2 cup water and add to the food processor with butter, sugar, eggs, vanilla and flour. Whizz to combine. Add sultanas or raisins, and walnuts if using, and stir with a spoon or pulse several times to just combine (don't whizz them or they will break up).
Pour into prepared cake tin and bake until a skewer inserted into the centre comes out clean. (about 1 hour)⏱ 60 min
Orange lightning cake will keep in an airtight container for up to a week or can be frozen.