1/4 c. wine vinegar
1/4 c. dry white wine or vermouth
1 tbsp. minced shallots or scallions
1/2 tsp. dried tarragon
1/4 tsp. salt
1/8 tsp. pepper
3 egg yolks
1 stick butter

Boil the vinegar, wine, and herbs and seasonings in a small saucepan until liquid has reduced to about 2 tablespoons. Place egg yolks, vinegar mixture, in a jar of blender. Cover and blend at high speed for 30 seconds. Uncover, and still blending at high speed, start pouring in the hot butter by droplets. The heat of the butter warms the egg yolks and by pouring very slowly you are giving the yolks time to absorb the butter. When about 2/3 of the butter has gone in, sauce should be a thick cream and you can pour a little more quickly.