To prepare potatoes, place potatoes in a Dutch oven, and cover with water. 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 sugar, 1 teaspoon salt, and vanilla. Beat with a mixer at medium speed until smooth. Add eggs; beat well (mixture will be thin). Scrape mixture into a 13 x 9-inch baking dish coated with cooking spray.
To prepare topping, sprinkle miniature marshmallows over top of casserole. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, 1/4 cup sugar, and 1/4 teaspoon salt in a medium bowl. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Stir in pecans, and sprinkle over potato mixture and marshmallows. Bake at 375° for 30 minutes or until golden brown.