Instructions

• Cut 1 stick unsalted butter into pieces, place in a small saucepan and melt over low heat until just melted — do not let it brown or simmer. Remove from heat and keep warm.
• Meanwhile, put 3 room-temperature egg yolks, 4 teaspoons lemon juice, ¼ teaspoon salt, and a pinch of pepper into a tall, narrow measuring jug that the head of your immersion blender fits into.
• Insert the immersion blender into the jug and blend the egg yolk mixture for about 30–60 seconds, until it lightens in color and is evenly combined.
• With the blender running on its lowest speed and the blender head at the bottom of the jug, very slowly drizzle the hot melted butter in a thin, steady stream into the egg mixture. Continue until all the butter is incorporated.
• As the sauce emulsifies and thickens, slowly raise and lower the blender through the sauce to fully combine and reach the desired consistency. Blend briefly to finish.
• Taste the sauce — it should be slightly lemony and lightly seasoned. If desired, add small additional amounts of lemon juice, salt, or pepper to taste, blending briefly after each addition.
• Serve immediately. If you must hold the sauce, place the jug in a bowl of warm (not hot) water for up to one hour and reblend briefly before serving if needed.