Clean the kumquats with salt water. This removes impurities and helps lessen the bitterness.

Remove the green stems, slice the kumquats in half, and remove the seeds with a fork or the tip of a knife. It is perfectly fine if there are one or two seeds you forgot to remove since it is still edible. However, since it is the source of bitterness, it is best to make sure that all are taken out.

In a pan, heat the chopped kumquats with rock sugar. Keep stirring over medium heat until the rock sugar melts and the mixture thickens. Eventually, it should become sticky and syrupy, submerging the chopped kumquats. Do not leave the kumquats unattended. Otherwise, you risk burning the sugar and hardening.

Once the kumquats become sticky, arrange them on a baking tray lined with parchment paper. Bake at 100 degrees Celsius for an hour, or until dry to the touch. Do not overload the tray, and make sure there are gaps between each kumquat. Arrange the kumquats in a single layer, not stacked. This cooks the kumquat evenly and ensures it dries up.

Finally, coat it evenly with granulated sugar. Enjoy it while warm and soft or let it chill in the fridge. Store it in the fridge in an airtight container and consume it within a week.