This tangy, sweet mustard sauce for corned beef is super easy to prepare with basic pantry ingredients to complete your St. Patrick's Day meal.
Melt butter in a saucepan over low heat; remove from heat.
Whisk brown sugar, white sugar, mustard, egg, salt, and pepper together in a medium bowl. Whisk in vinegar until well combined. Stir mixture into melted butter in the saucepan.
Stir sauce over medium heat until boiling. Reduce heat and simmer for 3 minutes more.