Drain can of liquid into separate bowl. Take 2 cups of liquid and pour into sauce pan.
Add butter to the liquid in saucepan to start melting. Once melted, add the sugars.
Bring to a boil constantly stirring until liquid thickens (can be to your liking, but I prefer semi-thick)
Pour the drained potatoes into a casserole dish. Add the thickened liquid, mash potatoes into chunks or as small as you prefer. Top with the marshmallows, press into top of potatoes so liquid can get to the mallows.
Bake at 350 until marshmallows are golden and potatoes are bubbly. Be careful, the marshmallows can burn.
Once done, take out of oven, let sit for about 5 minutes, then serve.