Grease an 8-inch round cake pan, line the bottom with parchment paper and grease paper.
Cut the orange into quarters, remove any seeds and blend in a food processor until smooth, scraping down sides at least once.
Dissolve baking soda in ½ cup water and add to the food processor with butter, sugar, eggs, vanilla and flour. Process to combine. Add pecans and stir with a spoon or pulse a few times to incorporate.
Pour batter into prepared cake tin and sprinkle with coarse baker’s sugar.
Bake 50-60 minutes, until a skewer inserted into the center comes out clean.
Cool on a rack for 15-20 minutes, run a knife around edge of cake and invert on to cooling rack to cool completely.