In a saucepan on the stovetop, whisk together the sugar, cornstarch, water, and lemon juice until the mixture is not lumpy. Cook over a low to medium heat and stir occasionally until the mixture begins to thicken. Stir constantly once the mixture is thickening. Remove from heat when the sauce is the consistency you desire.
Add in the butter and stir until melted. Serve warm or cold over muffins, sweet breads, or ice cream.