Hasselback Potatoes

4 large potatoes
4 tablespoons

melted butter, olive oil, duck fat, bacon fat, coconut oil, or a mix
Salt
Pepper
Optional extras: minced fresh herbs, spices, grated cheese, breadcrumbs, panko crumbs
Heat the oven to 180°C. Arrange a rack in the bottom third of the oven and heat to 180°C.
Wash and dry the potatoes. Scrub 4 potatoes clean and pat them dry. Alternatively, you can peel the skins off.
Cut slits in the potatoes, leaving the bottom intact. Cut parallel slits into each potato, stopping just before you cut through so that the slices stay connected at the bottom of the potato. You can rest the potato in a large serving spoon (or on 2 wooden chopsticks) and use that as a guide for when to stop slicing — slice straight down and when your knife hits the edge of the spoon, stop slicing.
Brush the potatoes with half the fat. Arrange the potatoes in a baking dish. Brush the potatoes all over with 2 tablespoons of butter or other fat, including the bottoms.
Season with salt and pepper. Sprinkle the potatoes generously with salt and pepper.
Bake for 30 minutes, then brush with more fat. Bake the potatoes for 30 minutes. At this point, the layers will start separating. Remove the pan from the oven and brush the potatoes again with the remaining 2 tablespoons of fat — you can nudge the layers apart if they're still sticking together. Make sure some of the fat drips down into the space between the slices.
Bake another 30 to 40 minutes. Bake until the potatoes are crispy on the edges and easily pierced in the middles with a paring knife, 30 to 40 minutes. If you're adding any extras, stuff those into the slits and sprinkle over the top 5 to 10 minutes before the end of cooking. (Total baking time is 60 to 70 minutes for average potatoes; if your potatoes are on the small side or are larger, adjust cooking time accordingly.)
Serve immediately. These potatoes are best straight from the oven while the edges are at their crispiest.