Spray the inside crock of a slow cooker with cooking spray.

Drain the canned yams, transfer them to a large mixing bowl, and mash them with a potato masher or the back of a wooden spoon. Add ¼ cup melted butter, 2 tablespoons brown sugar, egg, cinnamon, and salt. Mix well.

Spread the mashed sweet potato mixture evenly in the crock. 

In a small bowl, combine the remaining brown sugar and butter, chopped pecans, and flour. Spread the mixture evenly over the top of the sweet potatoes.

Cover and cook on low for 3-4 hours. Sprinkle on the marshmallows in the last hour.