Place the clementines in a pot and cover with cold water. Bring to a boil, then simmer for 50 minutes and drain.: As the clementines simmer, the peels soften and the membranes mellow, creating a cooked fruit that blends into a silky puree. You will notice a fragrant citrus steam and a deeper, sweeter scent than raw fruit; that gentle scent signals the peel oils have released into the flesh. This long simmer also reduces bitterness that can lurk in the pith. One common mistake is undercooking them, which leaves a sharper, more bitter edge; make sure they are tender when you pierce them with a fork. After draining, allow them to cool fully so they are easier to handle and so the batter temperature stays moderate when mixed.
Preheat oven to 180ºC/160°C Fan/360ºF. Grease and line with parchment paper a 20 cm / 8-inch cake tin. Arrange a baking rack onto the middle shelf of the oven.: Heating the oven in advance stabilizes the bake so the cake starts rising immediately on contact with heat. Use the middle rack for even air circulation, which helps the cake set uniformly and prevents a sunken center. Greasing and lining the pan ensures easy release later, especially since the crumb stays tender. A frequent oversight is placing the pan too low or too high in the oven, which can lead to uneven browning; middle placement is safest.
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.: The texture you want here is smooth and velvety, with no large pieces remaining. As you process, the sound will shift from chunky chopping to a steady hum, and the puree should have a glossy sheen. This smoothness is crucial because it disperses moisture evenly through the batter, helping the cake set with a tender crumb. If seeds remain, they can create unwanted bitter specks, so double check; a common mistake is skipping the seed removal and finding tiny bitter bites in the finished cake.
In a large bowl whisk the eggs with the sugar until light and pale, about 3 minutes.: When you whisk the eggs and sugar to a pale, slightly ribboning stage, you incorporate air that lifts the cake. Visually, the mixture will double slightly in volume and turn a soft lemon color; it should fall back into the bowl slowly when you lift the whisk. The tactile clue is a silky, slightly thickened liquid. Not whipping long enough results in a denser cake, while overwhipping can make the mixture collapse later; aim for that gentle volume increase.
Add the whisked eggs, almond flour, baking powder, and optional orange zest in the food processor. Blitz for 5 - 10 seconds until combined.: This short pulse keeps the batter homogeneous without overworking the almond flour . You should hear quick, even pulses and see the batter just come together into a slightly glossy mass. Overprocessing can break down air pockets and make the texture heavy, so brief bursts are best. If the batter looks grainy, a couple more seconds are fine, but avoid long continuous blending which can heat the batter and affect lift.
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.: During baking you will notice a fragrant, toasty citrus aroma and the top will change from pale to a warm golden tone. The sound of the oven is steady; there should be no large oven eruptions like smoke. The toothpick test tells you when the interior has set; if it comes out with a few moist crumbs, that is okay as they will firm slightly while cooling. A common slip is removing it too early, which yields an underbaked middle, so err on the side of checking a couple times toward the end and tenting with foil if the top browns ahead of doneness.
Remove the pan from the oven and arrange it over a cooling rack, but leave the cake in the pan to cool.: Cooling in the pan helps the cake settle without cracking, preserving its delicate structure. You will notice the aroma mellow and the surface become slightly matte as steam escapes. If you try to unmold it while hot, it may break apart because the crumb is still fragile. The cake will also firm up a bit as it cools, so patience prevents a ruined presentation; wait until it is mostly at room temperature before attempting to lift it out.
Mix the glaze ingredients until you have a thick but spreadable glaze. Pour it over the top of the clementine cake and serve.: The glaze should be glossy and slightly viscous so it spreads without running off the edges. When you spoon it on, you should see a smooth ribbon that quickly flattens into a thin coat, highlighting the cake’s surface. This final touch brings a concentrated citrus brightness and a sweet sheen. A common error is thinning the glaze too much with liquid, making it run; add juice sparingly to keep a spreadable consistency.
