Maris Piper or King Edward are the classic potato varieties to use for roast potatoes. Being on the floury side of varieties (as opposed to waxy) they will be nice and fluffy in the middle, crispy on the outside.
Peel and boil with a good amount of salt in the water - this will help season them throughout. Once a knife goes in easily and they are cooked to the middle. Strain them in a colander then pour into a roasting tray. Spread them out to allow them to steam dry. The aim here is to remove as much moisture as possible as dry = crispy. You can do this a day ahead, leaving the boiled potatoes (once cooled) uncovered in the refrigerator overnight.
The fridge is a dry atmosphere and will remove moisture from food (think about how a piece of cheese looks if left unwrapped after a day or so), which is perfect to dry your potatoes and get them extra crispy when roasted. This is the same principal Heston Blumenthal used when creating his famous triple cooked chips.
Once the potatoes are steamed dry and cool, spoon 100ml of Wagyu Dripping into a clean roasting tin and heat in the oven at 200°C for 5-10 minutes. Remove and pour in your cold potatoes and coat well - a little rapeseed oil drizzled over, if needed, works very well. Season with salt and a few sprigs of rosemary. I favour also adding an unpeeled clove or 2 of garlic to infuse its flavour during cooking.