For the kumquats sheet cake, first wash the kumquats well, dry them and cut off the stem end. Cut into thin slices and remove the seeds if necessary. Bring to a boil with the remaining ingredients. Set aside and allow to cool.
Mix all ingredients for the batter. Fold in the kumquats.
Line a baking sheet with parchment paper and spread the batter. Place the kumquat cake in the oven preheated to 180 °C top/bottom heat for about 20 minutes.