1 14-oz can sweetened condensed milk
Empty the can into a large (2-1/2 quart or larger) glass bowl, and cover with plastic wrap (cling film), keeping a tiny bit uncovered to prevent excess steam build-up.
Microwave on medium power (I used level 5 on a 10-level appliance) for 2 minutes. Remove, stir with a wire whisk, and recover. Cook on medium for 2 more minutes. Remove, stir with a whisk, recover.
Then, in increments of 2-1/2 minutes, cook (on medium power) for another 10 minutes, stirring between each interval. After the first two stirs, you'll notice that the milk bubbles and foams up as it expels moisture. Then, with each stirring, the milk will be thicker and more caramel colored.
If after the 10 minutes, you like the color and consistency, stop! If you'd like a thicker sauce, continue cooking in 1-minute increments for another 2-3 minutes.
Remove from the microwave, and let cool before packing in a glass jar, or use right away as a topping for ice cream.