Melt the butter in a small pot at low heat (but do not brown).
Take the pot off the stove. Skim off the white whey flakes with a tablespoon. Let the butter in the pot cool down a little.
Peel and finely chop the shallot. Bring to the boil in a small pot with vinegar and 2 tablespoons of water. Pour through a fine sieve into a small stainless steel bowl or pot.
Heat the water in a pot - in which you can hang the bowl - to about 60-70 °C. The temperature is best measured with a roast thermometer. Hang the stainless steel bowl in the hot water. Beat the egg yolk and 1 tbsp. cold water.
Whisk over a hot water bath for about 10 minutes until a thick cream is formed. Remove bowl from water bath and continue beating for approx. 1 minute.
Stir the melted butter first drop by drop, then in a thin stream with a whisk into the egg yolk cream. Season to taste with salt and pepper. Best served immediately with the asparagus, as the sauce will become firm again when it cools down!