Potatoes can be boiled, drained, and then mashed OR cooked in a slow cooker or Instant Pot (my favorite method - see note for directions!).
Preheat oven to 375 and mash your cooked sweet potatoes.
Add eggs, vanilla, 1/2 cup melted butter, brown sugar, salt, and milk (or heavy cream) and mix until smooth. (I like my potatoes to have a bit of texture so I don't whip them for too long).
Spread potato mixture in the bottom of a greased 9x13 baking dish.
In a bowl combine brown sugar, flour, 1/3 cup melted butter, and pecans and mix well (with a spoon, not a mixer). Sprinkle mixture over potatoes (it's okay if potatoes aren't completely covered).
Bake uncovered for about 20-25 minutes until pecans are caramelized.
Sprinkle marshmallows evenly over pecan mixture. Return to oven for another 5-7 minutes or until marshmallows are puffy and lightly browned. Serve and enjoy!