Position racks in the middle and upper third of the oven; preheat to 400 degrees F. Scrub the potatoes and dry well. Bake directly on the middle oven rack until easily pierced with a knife, 45 to 50 minutes. Remove from the oven; let cool 5 minutes.
Meanwhile, melt 2 tablespoons butter in a medium skillet over medium heat. Add the ham and leek; cook, stirring occasionally, until the leek is wilted, about 6 minutes. Add the cauliflower and chicken broth; season with salt and pepper. Bring to a simmer, cover and cook until the vegetables are tender, 4 to 5 minutes. Uncover and lightly mash the cauliflower. Increase the heat to medium high and cook until the liquid evaporates, 2 to 3 more minutes.
One at a time, hold each potato with a kitchen towel and halve lengthwise. Scoop the flesh into a large bowl, leaving a 1/4-inch-thick shell. Mash the flesh with 2 tablespoons butter, the heavy cream and cheese until almost smooth and the cheese melts. Stir in the ham mixture, parsley and mustard; season with salt and pepper. Melt the remaining 2 tablespoons butter; brush the potato skins with some of the butter and season the insides with salt and pepper. Set on a baking sheet. Add the breadcrumbs to the remaining melted butter.
Mound the filling into the potato skins and sprinkle the tops with the breadcrumb mixture. Return to the oven on the top rack and bake until the filling starts browning and the skins are crisp, 18 to 20 minutes.