Preparation

1. In a medium size stainless steal of heatproof glass bowl whisk together the egg yolk and lemon juice until light and slightly frothy. Create a double boiler so that the bowl fits nicely over a saucepan. Fill the bottom of the saucepan with a small amount of water and bring to a low simmer - but just make sure that there is not too much water that it touches the bottom of the bowl. Set the egg yolk mixture over the saucepan and whisk in the pieces of butter one at a time until incorporated. Continue to whisk until thickened as desired. Remove bowl from heat and season with a pinch of salt and tiny dash of cayenne pepper. If it looks like it is getting too thick too quickly - remove the bowl from the heat (use a towel or oven mitt to avoid getting hurt from the steam) and you can thin it by whisking in a few teaspoons of hot water.
2. Place warm or chilled steamed asparagus spears onto serving plates and drizzle with the sauce. This rich and delicious sauce is also wonderful with broccoli, artichokes, poached fish and of course the classic Eggs Benedict.