Nutrition per serving may change if servings are adjusted.
1 tablespoon cornstarch
1 1/2 teaspoons dry mustard, preferably Colman’s
1 teaspoon sugar
1/2 teaspoon salt
Pinch of cayenne pepper
3/4 cup buttermilk, divided
1 large egg
2 tablespoons lemon juice
1 tablespoon extra-virgin olive oil
Whisk together cornstarch, mustard, sugar, salt and cayenne in a medium saucepan. Add 1/4 cup of the buttermilk and whisk until you have a smooth paste. Add egg and the remaining 1/2 cup buttermilk and whisk until smooth.
Set the pan over medium-low heat and cook, whisking, until the mixture comes to a simmer and thickens. Continue to whisk for 15 seconds, then remove the pan from the heat.
Whisk in lemon juice and oil. Transfer the mayonnaise to a small bowl and press a piece of plastic wrap directly on the surface to prevent a skin from forming. Let cool.
Make Ahead Tip: Cover and refrigerate up to 3 days. Whisk before using.