Meanwhile whisk egg yolks and sugar until pale, almost white.
Stir egg and sugar mixture into hot cream stirring well on low heat until thick enough to coat the back of a spoon. Remove from heat.
Take your measuring spoon and start small. If you have Nigella’s measuring spoons, mix 1 “pinch†of wasabi with 1 “pinch†of hot water. If you are using regular measuring spoons, start with a 1/4 teaspoon of wasabi blended with 1/4 teaspoon of hot water. Add this to the custard and taste. Keep adding wasabi and water in these tiny increments until you feel that its wasabi-ish enough. I can’t say that my measurement would be hot enough for most. For me, 3 “pinches†was enough. For my husband, 4 1/2 “pinches†.
Freeze until firm and once ready to serve, ripen in the fridge for 5-10 minutes.
Cream acts as the heavy vehicle carrying volatile heat compounds across the palate without scorching them into bitterness. Dairy fat coats the tongue first, delaying the sharp nasal impact characteristic of Japanese horseradish until the base begins to melt. Heating that cream requires constant attention in a heavy-bottomed saucepan to prevent scorching the milk solids on the base of the metal. Whisking egg yolks with granulated sugar until the mixture turns pale introduces air pockets that later aid the emulsification process. Cooking this custard over low heat demands a reliable visual test rather than a strict timer. Pull a wooden spoon through the steaming liquid and drag a finger across the back; if the line holds clean without running, the proteins have coagulated sufficiently to thicken the base. Wasabi loses potency rapidly when exposed to prolonged heat, which dictates a methodical blending technique. Dissolve the wasabi paste in a few drops of hot water in a separate small bowl before incorporating it incrementally off the heat. This pre-dilution step prevents green lumps from forming in the smooth yellow custard. Taste the mixture frequently as additions increase, keeping in mind that freezing dulls piquancy by roughly thirty percent. Transfer the finished base to a shallow metal container to accelerate the initial chill in the freezer compartment. Dense frozen desserts often freeze too hard for immediate scooping after several hours. Allow the container to rest in the refrigerator for five to ten minutes before serving to restore a pliable texture. Storing leftovers beyond forty-eight hours may result in icrystal formation, so consume the batch promptly for optimal mouthfeel.