Note: I've made this several times (though never the same twice) and it tends to be rather sweet, almost like a desert.. but that could be my fault..

2 29oz Can sweet potatoes in syrup
(keep juice from ½ can)
1 ½ Cup Sugar / Splenda
2 teaspoons cinnamon
½ stick butter (softened)

Blend ingredients until they have a consistency similar to mashed potatoes.
Pour mix into casserole dish
Top with marshmallows

Bake @ 375 for approx 45 mins

Alternately, use no syrup and less sugar and add 1 – 1 ½ cup liquor (ie: Crown Royal )