2 tablespoons water
large pinch salt
4 ounces unsalted butter, chilled and cut into bits (if your butter is salted, omit the pinch of salt)
Put the water in a small heavy-bottomed saucepan, add the salt, and bring it to a simmer over very low heat. When it begins bubbling, toss in a couple bits of butter and whisk until barely melted. Continue whisking in a few bits at a time until all are incorporated ant eh butter is the consistency of this cream. If it begins to overheat and separate, immediately take it from the heat and whisk in a few bits of butter to cool it slightly.
As soon as all the butter is incorporated, remove the sauce from the heat and pour it into a warm sauceboat. (I poured it over my warm carrots and my warm Japanese turnips. Then after discovering that the turnips were slightly overcooked and made the sauce a bit watery, I mashed them and added a tad of Tomme cheese and they were divine. Seriously. DIVINE.) This would be a fabulous sauce to serve over roasted daikon as well.