Sweetened condensed milk is a bit tricky to cook in a microwave. Although sweetened condensed milk keeps longer, it is still a type of milk that will curdle due to the intensity of the heat when microwaved. The key to this is avoiding extremely high wattage power settings and always placing the milk and other ingredients in a bowl that is safe for microwaving. You can generally tell when a bowl is not meant to be microwaved since even a lower power setting with only seconds inside the oven will make the bowl painfully hot to the touch. This also means that the ingredients in the bowl will easily scorch, and in the case of milk, will curdle very quickly.