Add boiling water to a pot (so its about ¾ full), add the orange, and cook on high for 1 hour until the orange becomes tender. (Don’t cover the pot). If orange develops some brownish marks from where it is in contact with the pot, this is normal.
Once orange is soft and tender, drain the water and let it cool. Slice orange in half and remove seeds, but keep the skin on.
Preheat fan oven to 160 degrees C/320 degrees F and grease a round 20 cm spring form tin with butter all round the sides. Place parchment paper on top of base plate and lock in place with the sides
Add orange to a food processor or blender, along with granulated sweetener and blitz to a puree.
In a large mixing bowl, add the almond meal, eggs, baking powder, and orange mixture. Use a spatula to gently mix together until combined, making sure all the almond meal is blended in.
Pour mixture into prepared cake tin and bake for 50 - 55 minutes until golden. Smooth over the top of the cake with a butter knife. Cake is cooked when a skewer is inserted into the centre and comes out clean or with just a few crumbs.
Let cake cool in the tin for 15 minutes. Run a butter knife around the edge of the tin to loosen cake, gently release spring from tin so that sides come away. Slide cake off base, leaving paper in tact and onto a wire rack to cool further at room temperature (covering with a towel)