I have this distressing habit of throwing stuff together to see if it works. When it does, I have to guess what my proportions were so I can repeat my success.
So I've "thrown" this together a few times and it's always yummy. This is my best guess for amounts.
¼ cup butter
½ cup brown sugar
½ cup mini marshmallows
Melt those together in a saucepan.
I'm too impatient, but almonds or pecans and a tsp of vanilla, mixed in after the sauce is off the heat, would be really yummy.