Bearnaise Sauce

1/2 cup white wine vinegar
1 tablespoon tarragon
1 shallot finely chopped
3 egg yolks
1 cup melted butter

Simmer the vinegar, tarragon and shallot on low until reduced to about 1 tablespoon. 
Strain and place in a tall narrow container.  Just big enough to fit the head of an immersion blender. Melt butter in a small sauce pan until hot.  Add the three egg yolks to container.  Place the blender in the container.  While the blender is running, slowly drizzle the hot butter into the egg mixture.  Do this slowly so that the butter and the eggs emulsify.  (Like making mayonnaise.) 

When the sauce is thick, season with salt and pepper and more tarragon to taste.