Boil the potatoes in a generous amount of water until very tender. Drain well and mash thoroughly. Whip in the buttermilk and 1/4 cup of butter.
Season with salt and pepper and mix in 1 cup of the cheese. Lightly butter a shallow-sided, 8-cup capacity casserole. Spread the potatoes into the casserole (if baking later, cool to room temperature, cover and refrigerate until needed).
Preheat the oven to 375 degrees F. Place the remaining cheese in a bowl and mix in the breadcrumbs and herbes de Provence. Sprinkle this mixture over the potatoes and then drizzle with a little melted butter. Bake, uncovered, for 25 to 30 minutes, or until heated through and golden.