For safety reasons, heating the unopened can (an old cooking method) is NOT recommended.
1 (14 ounce) can sweetened condensed milk
Oven Method: Pour sweetened condensed milk into 9-inch pie plate. Cover with aluminum
foil; place in larger shallow pan. Fill larger pan with hot water. Bake
at 425 degrees F for 1 hour or until thick and caramel-colored. Beat
Stovetop Method: Pour sweetened condensed milk into top of double boiler; place over
boiling water. Simmer over low heat for 1 to 1/2 hours or until thick
and caramel-colored, stirring occasionally. Beat until smooth.
Microwave Method: Pour sweetened condensed milk into a 2-quart glass measuring cup. Cook
on 50% power (medium) 4 minutes, stirring briskly every 2 minutes until
smooth. Cook on 30% power (medium-low) 20 to 25 minutes or until very
thick and caramel-colored, stirring briskly every 4 minutes during the
first 16 minutes and every 2 minutes during the last 4 to 10 minutes.