To make our copycat Whataburger sweet and spicy pepper sauce, you will need a medium saucepan over medium heat. Into the saucepan add cornstarch, water, vinegar, corn syrup, egg yolk, and buttermilk. Stir this mixture until it is combined, then continue stirring the mixture over medium-low heat until the mixture begins to thicken and reaches at least 165°F.
Once the mixture thickens and reaches 165°F, remove the mixture from the heat and add salt, mustard, onion powder, garlic powder, lemon juice, parsley, sugar, and chipotle pepper or jalapeño peppers. Stir to combine, then allow the mixture to cool before adding to your burgers.
If desired, you can add it to your burgers while still warm, or you can prepare the sauce ahead of time and place it in the refrigerator to chill the sauce before adding it to your burgers.