Rinse diced potatoes in cold water, drain well, and pat dry using a paper towel. While potatoes are drying begin to melt butter in a large cast iron pan.
Place potatoes into the pan, and season to taste with salt and pepper. Cook potatoes over medium heat for 10-12 minutes, or the potatoes become fork tender.
Remove the lid from the pan, and increase the heat to medium high. Sear and turn potatoes until they have browned and are roasted on all sides.