Place the sugar and yuzu zest (if you have any, if you are using store bought yuzu juice proceed to the next step) in a medium heat proof bowl. Use your fingers to rub the yuzu zest into the sugar.
Set the bowl over a small saucepan 1/4 filled with water over a low heat. Ensure the water won’t touch the bowl.
Add all the curd ingredients to the heat proof bowl and whisk to combine. Place the bowl over the heat.
It will take a few minutes for the ingredients to emulsify together so keep whisking until they do. Once they do, continue whisking for 5-10 minutes, or until the curd has thickened and coats the back of a spoon. As soon as it does, remove it from the heat. If you’re cautious, you can plunge the bowl base into an ice or cold water bath to stop the curd from cooking.
If you hear the water bubbling at any point during cooking, turn the heat down. Low and slow temperatures create a lovely thick curd.
Cover the curd with plastic wrap, flush to the curd itself so it doesn’t develop a skin. Place in the fridge and allow to cool completely (if you’re not using it straight away) before decanting into a clean, airtight container. Use it within a week or freeze it to preserve it for longer.
If you find your curd has little white lumps (cooked egg white) simply pass it through a fine mesh sieve once the curd is cool.