Melt butter in a small saucepan over low heat. Cook, swirling the pan, until the butter turns golden, 30 to 60 seconds. Pour into a small bowl and set aside.Advertisement
Whisk 1/4 cup buttermilk, cornstarch, salt and cayenne in a heavy medium saucepan until smooth. Whisk in egg along with the remaining 1/2 cup buttermilk.
Set the pan over medium-low heat and cook the sauce, whisking constantly, until it comes to a simmer. Cook, whisking, for 15 seconds. Remove from heat and whisk in lemon juice and the reserved butter. Serve hot or warm.
To make Sauce Moutarde, add 2 teaspoons Dijon mustard. For Sauce Maltaise, use 1 1/2 tablespoons orange juice in place of the lemon juice.
Make Ahead Tip: Cover and refrigerate for up to 2 days. Reheat in a microwave on High for 1 to 1 1/2 minutes, stirring once.
Tip: No buttermilk? You can use buttermilk powder prepared according to package directions. Or make “sour milk”: mix 1 tablespoon lemon juice or vinegar to 1 cup milk.