No water needed!
Peel the yams and cut into 3/4 inch slices. Place the yams in an 8 quart stock pot, cover with brown sugar, sugar, and butter. Cook over low to medium low heat until the yams are fork-tender (about 20 to 25 minutes, do not overcook). Water is not needed ~ the yams cook in the sugar and butter without water, as the yams themselves give off their own juices. Do not add the sweetened condensed milk yet.
Once the yams are fork-tender, use a masher or hand mixer to blend all that sugary, juicy goodness with the yams ~ No need to remove any of the liquid, it will all blend perfectly.
Mix in, to taste, a couple of tablespoons of sweetened condensed milk. This will give your yam mixture an ooey, gooey texture.
You may also add cinnamon and/or nutmeg to taste.
Spoon into a casserole dish and cover lightly with marshmallows and bake uncovered at 350° until the marshmallows are a light golden brown and ENJOY!
If you double this recipe, use a really big pot, otherwise, cook time is longer.
To make this ahead of time, place the covered casserole in the fridge without the marshmallows. Once you're ready to bake, first place in the microwave on low or defrost just to take the chill off and then add the marshmallows and pop it in the oven at 350°.