Set the cranberries, orange juice, orange zest and sugar in a large pot over medium heat. Cook for about 15-20 minutes, until most of the cranberries have burst and all are soft. Turn off the heat.
Use an immersion blender (or transfer the mix to a blender or food processor) to blend the cranberries into the juice. Through a fine mesh sieve, pour the puree into a bowl (or back into the pot if you used a blender). Press to squeeze out all the juice from the berries.
Crack all the eggs into a bowl or into the cooled pot if it's empty, (we don’t want the heat of the pot to start cooking the eggs before they are whisked in).
Pour the curd back into the pot over the eggs, whisking as you pour and set it over medium low heat. Cook until the curd thickens and covers the back of a spoon. On a thermometer it should reach 165 F. The curd will thicken on the bottom first, be sure to stir it every 30 seconds so it doesn’t burn on the bottom.
Set the butter in a large bowl and set a fine mesh sieve (you can reuse the one you used earlier, just rinse it well) over the butter. Pour the curd through the sieve, pressing to extract all the juice. This will remove any unwanted cooked egg white bits.
Stir the butter into the curd until it fully melts. At this point you can transfer to an airtight container and store in the fridge for a few hours or up to a week. You can also freeze the curd for 2 months.