Cut the butter into small cubes. Set aside and let it get to room temperature.
Prepare the double broiler by filling a medium pot with 1 – 2 inches of water. Set a medium heatproof bowl (glass or stainless steel) over. Check the bottom of the bowl to make sure it didn’t touch the water in the pot. Remove the bowl. Bring the water to a full boil over medium heat.
Once the water is boiling, reduce to low heat and keep at a simmer. Add egg yolks, sugar, zests, juices and salt to the bowl. Set it over the pot of simmering water. Whisk constantly until thicken like sour cream, about 10 minutes. Curd will thicken more as it cools. Remove from heat. Add the butter and whisk until smooth.
Transfer the curd to a small glass container or bowl. Cover with plastic wrap by pressing down to the surface of the curd. This will prevent a skin from forming on the curd. Refrigerate until cool.
When ready to serve, remove the plastic wrap and give a few stirs. It’s ready to be used.
Notes
Make sure to use only glass, ceramic or stain steel bowls for this recipe. Citrus acid in citrus juices can have a chemical reaction with reactive metals, like aluminum, and copper. These metals will give your juice and curd a metallic taste.
You can substitute the mandarin oranges with any tangerines, or even oranges.
When whisking the egg mixture, it will get foamy. It’s normal. Once it’s thickened, it will be creamy but still has tiny bubbles.
Once the curd is cooled, you can remove the plastic wrap completely. A skin will not formed at this point. Just cover with a lid (or with a plastic wrap like how you normally do it).
For storage, the curd can last for about 2 weeks in the fridge. It can also be frozen and kept up to 3 months. Thaw in the fridge overnight before using.