Preheat the oven to 200°C.
Place a potato between two chopsticks on a chopping board. Using a sharp knife, cut thin slices across the potato. The chopsticks will prevent the knife from cutting all the way through. For the first few slices avoid cutting through completely, then continue slicing while keeping the base intact. Repeat with all the potatoes.
In a bowl, mix together the butter, gochujang, garlic, salt, mushroom seasoning powder, black pepper, and sesame oil until well combined.
Place the prepared potatoes onto a baking tray lined with baking paper.
Spoon about 1 tablespoon of the gochujang butter mixture over each potato, making sure some of the butter goes between the slices.
Bake in the oven for 50 to 60 minutes until the potatoes are crispy on the outside and tender in the centre.
To serve, sprinkle with chopped spring onion and drizzle with chilli oil.