This was the hardest part.. First, I had to find satsumas. Found them in the third grocery store I tried (Trader Joe's). Then I had to get some gelatin sheets, which I had never used before. Got those off Amazon.First step, cut the gelatin sheets in ~1 inch strips and place them in cold water to start to soften. Combine the eggs, sugar, and juices in either a double boiler, or a glass bowl set over a pot of simmering water. Cook, whisking constantly, until it has thickened and a thermometer in it reads 82C.
Let it cool for about a minute, while whisking. Squeeze the water out of the softened gelatin and add it to the curd, whisking for about another minute until it is dissolved. Strain the curd through a very fine strainer.
Use an immersion blender on low to mix in all the butter slowly, a few pieces at a time. Once all the butter is incorporated the curd is ready. You can use it immediately or refrigerate. I made it the day before and put a sheet of plastic rap on the surface of the curd to stop it from forming a skin.