The perfect accompaniment to asparagus...
Begin by placing the egg yolks in a small bowl and season them with a pinch of salt and pepper. Then place them in a blender and blend them thoroughly for about 1 minute. After that, heat the lemon juice and white wine vinegar in a small saucepan until the mixture starts to bubble and simmer. Switch the blender on again and pour the hot liquid on to the egg yolks in a slow, steady stream. After that, switch the processor or blender off.
Now, using the same saucepan, melt the butter over a gentle heat, being very careful not to let it brown. When the butter is foaming, switch the blender on once more and pour in the butter in a thin, slow, steady trickle; the slower you add it the better. (If it helps you to use a jug and not pour from the saucepan, warm a jug with boiling water, discard the boiling water and then pour the butter mixture into that first.) When all the butter has been incorporated, wipe around the sides of the processor bowl or blender with a spatula to incorporate all the sauce, then give the sauce one more quick burst and you should end up with a lovely, smooth, thick, buttery sauce.