Scrub the potatoes and dry well. Bake directly on the middle rack in a preheated 400º oven for 45 to 50 minutes, or until easily pierced with a fork. Remove from oven and let cool 5 minutes.
Meanwhile, melt 2 Tbsp. of the 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 heat to medium high and cook until liquid evaporates, 2 to 3 more minutes.
Halve each potato lengthwise. Scoop the flesh into a large bowl, leaving a 1/4 inch thick shell. Mash the flesh with 2 Tbsp. 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 Tbsp. 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, about 18 to 20 minutes. Serve and top with sour cream, if desired.