Place the clementines in a pot and cover with cold water. Bring to a boil, then simmer for 50 minutes and drain.
Preheat oven to 180ºC/160°C Fan/360ºF. Grease and line with parchment paper a 20cm / 8-inch cake tin. Arrange a baking rack onto the middle shelf of the oven.
Once cool, cut the clementines in half and remove any seeds, then place them in the bowl of a food processor and blend until pureed and creamy, about 1 minute.
In a large bowl whisk the eggs with the sugar until light and pale, about 3 minutes.
Add the whisked eggs, almond flour, baking powder, and optional orange zest in the food processor. Blitz for 5 - 10 seconds until combined.
Pour the batter into a cake pan. Bake for 50 minutes until the surface is golden and a toothpick inserted into the centre comes out clean. After 40 minutes, you might need to cover the cake with foil to prevent the surface from browning too much.
Remove the pan from the oven and arrange it over a cooling rack, but leave the cake in the pan to cool.
Mix the glaze ingredients until you have a thick but spreadable glaze. Pour it over the top of the clementine cake and serve.