Microwave directions:
You will need a 2-1/2 quart microwave bowl for this recipe. If you don’t have one that big, make half at a time. Trust me on this unless you like having caramel all over your microwave.
Pour sweetened condensed milk into bowl. If you have a vented lid, put it on. If not, cover bowl with plastic wrap and poke a couple of holes in it so the steam can escape.
The gold standard will be a 1000 watt microwave, adjust accordingly.*
Set microwave to 50% power and microwave milk for 2 minutes. If you are adding the salt and cinnamon, add it now.
Remove, (use oven mitts please!) Stir and put it back in the microwave. Then nuke at half power in two and a half minute intervals, stirring in between each time, for a total of ten minutes. As you cook the milk, it will boil, expel steam and gradually turn a nice rich brown. Keep in mind, you will need a pretty thick sauce for the cookies. You may need to nuke the milk for another 2-3 minutes. You want a deep rich brown. It will thicken a little as it cools.
*If you have a 1200 watt microwave, set it at 40%. I have a tiny 600 watt microwave so I set the power at 60%.
The Other Method:
Requires a lot more time. First, remove the paper label from the can and place it in a heavy duty pot on its side. Fill pot with enough water to cover the can by 2 inches. Bring the water to a boil and reduce heat so the water is at a simmer.
Make sure the water level stays 2 inches over the can at all times!!!!!! Or the can might explode!!!! Why I use the microwave method, lol. This method comes from Serious Eats.
Bring the water to a boil and then reduce the heat to a simmer. Simmer milk for 2-3 hours CHECKING WATER LEVEL EVERY 30 MINUTES. Add hot water as needed. 2 hours will give you a golden caramel while 3 hours will yield a deep, dark caramel.
Remove can from water. Allow the can to cool completely before opening, about 4 hours.