Combine all ingredients except for butter in a heat proof bowl and whisk to combine.
Set a small pot filled with about 1 inch of water to simmer on the stovetop. Place your bowl over the pot to create a double boiler.
Cook your curd, continuously stirring, for about 5-7 minutes or until curd has thickened and become more opaque. This recipe produces curd that is spreadable and pourable, so it will not thicken to pudding consistency. Aim for gravy consistency.
Take the curd off the heat and stir in butter.
Cover with plastic wrap, touching the surface of the curd to prevent a skin forming, and put in the fridge to cool. You can transfer to a smaller jar if you'd like, but a bowl or shallow pan will allow the curd to cool faster.
Use curd with scones, toast, or other treats, keeping leftovers in an airtight container in the fridge.
Notes
Different brands of bottled yuzu juice will differ in tartness. The only way to tell how sour it will be is to taste it before cooking. Aim to have one a little more tart than lemon juice but not so much it'll burn your tongue. If you found your juice is weak in flavor (e.g. Yakami Orchards juice), you can substitute 50:50 yuzu juice and lemon juice to increase the acidity.
You can adjust sugar to your tastes. I found 3/4 cup sugar is a good level of sweetness to combat a very tart yuzu flavor, but you can decrease to 1/2 cup if you prefer a very tart curd.
Some metal whisks and bowls may leave your curd with a metallic flavor if it reacts to the acidic juice. You can use a silicone whisk or spatula and a glass bowl if your metal tools are reactive.