Transfer the mixture to a medium sized pot, and start to cook over low to medium heat. The key to a silky smooth curd is to cook it low and slow, so don't rush this process!
You'll want to whisk the curd constantly to avoid the egg from curdling. The curd will start to bubble and cook, it took me roughly 10-12 minutes for it to cook down to a thicker consistency.
Make sure you're scraping the sides and bottom of the pot to avoid burning at the edges! The temperature for the curd should be 175 F (80 C), it should be smooth and glossy, and coat the back of a spoon.
If you get the curd too hot, you run the risk of it becoming grainy so take your time here.
Once the curd is done cooking, transfer it right away to a heat safe bowl.
Cut the cold butter into small cubes, and add the butter, vanilla extract, and salt to the curd.
6 (85g) Tbsp unsalted butter, 1/8 tsp fine sea salt, 1/2 tsp vanilla
Whisk all of the remaining ingredients into the curd until all of the butter has melted and the curd is silky and smooth.