In a medium, heavy-bottomed saucepan, combine the milk and sugar. Stir thoroughly to blend.
Place the milk over medium heat and bring it to a boil. Reduce the heat and simmer gently, stirring from time to time. The milk and sugar will become "thin" and then eventually "simmer" back together and turn a light brown color. Continue to cook, stirring from time to time, until it the sauce is golden brown. This whole process takes about 25 to 35 minutes. Transfer to a bowl or jar to cool.