Boil potatoes in a large pot of water until fork tender.
Transfer potatoes to the bowl of a stand mixer (or a large bowl if using a hand mixer), add butter, and whip until smooth and butter has melted.
Add sweetened condensed milk and next eight ingredients (through salt), and whip for 30-45 seconds. Fold in marshmallows.
Transfer potatoes to a 9x13 casserole dish.
For the topping, in a measuring cup, melt remaining 2/3 stick of butter, and mix in flour and brown sugar. Only add water if you need to thin the mixture. Pour topping over casserole, spread thin to edges, and sprinkle pecans on top.
Bake for 40-45 minutes, until casserole is set and topping forms a slightly browned, flaky crust.
Recipe can be made a day in advance. Make the casserole as directed without adding the topping, cover tightly with plastic wrap, and refrigerate. Add the topping just before baking the next day.
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