Remove the zest from the oranges with a Microplane and set it aside. Juice the oranges and measure out a generous 1/2 cup of the juice. Taste it and add a splash of lemon juice if you feel it needs a little extra pucker.
Pour an inch of water into a medium saucepan and bring it to a simmer over medium-high heat.
Whisk together the juice, zest, egg yolks, and sugar in a heatproof bowl that will sit comfortably over the simmering saucepan.
Place the bowl over the saucepan. Switch to a spoon or silicone spatula and start stirring. Keep stirring until the curd thickens, coats the back of the spoon, and starts to cling to the sides of the pan between stirring. If the eggs look like they're starting to scramble instead of thicken, pull the bowl off the saucepan and turn the heat down.
When the curd has thickened sufficiently, remove it from the heat and stir in the butter.
Position a fine mesh sieve over a bowl and push the curd through. This removes the zest and any bits of scrambled egg (no matter how careful you are, you always end up with a few).
Scrape the finished curd into a jar and let it cool. Once it's down to room temperature, put a lid on the jar and pop it in the fridge.
This curd keeps for 10-14 days in the fridge. If you want to keep it longer, divide it into smaller jars and freeze them.