Preheat oven to 375° and grease 9x13 baking dish.
Peel and chop potatoes and cover with water in a large pot. Bring to a boil. Reduce heat, and simmer 20 minutes or until very tender. Drain; cool slightly.
Place potatoes in a large bowl. Add half-and-half, 3/4 cup brown sugar, 1 tsp salt, and vanilla. Beat with a mixer at medium speed, until smooth. Add eggs and beat well.
Pour mixture into prepared dish.
Sprinkle miniature marshmallows over top of casserole.
In a small bowl, combine flour, 1/4 cup sugar, and 1/4 teaspoon salt. Cut in butter with a pastry knife or 2 knives until mixture resembles coarse meal. Stir in pecans and sprinkle over potato mixture and marshmallows. Bake for 30 minutes or until golden brown.