Add the egg yolks and yuzu juice, along with zest if you have a real yuzu, to the bowl, season with a pinch of our smoked salt and freshly ground black pepper, and whisk together for a couple of minutes, over a low heat, until it starts to thicken a little. Then begin to add the melted butter, a teaspoon at a time at first, increasing to a steady trickle after the first few additions, whisking continuously. Keep adding the butter little by little until it’s all incorporated. Remove the bowl from the pan and rest on the work surface every now and then as you whisk to cool the sauce a little before returning to rest above the pan. Once all the butter is mixed you’ll have a sauce like mayonnaise. At this point, take a couple of tbsps of the hot water from the pan below the bowl, and whisk into the sauce until it’s the consistency of thick pouring cream. Turn off your heat, but leave the pan and bowl on the hob, the residual heat will keep your sauce warm.