Brown Butter Sauce:
In a small saucepan over medium heat melt and brown the butter;being careful not to burn it-cook until it turns light golden brown. Pour it into a heat proof bowl, leave the residue in the pan.
In a blender combine the egg yolks, mustard and lemon zest and lemon juice for only a few seconds. While the blender is on medium speed, slowly pour in the hot brown butter in a thin stream. The sauce will thicken quickly. Keep sauce warm over a bowl of hot water.