Preheat the oven to 200°C. Bake the potatoes until just tender, then leave them to cool enough to handle.
Halve the potatoes lengthways and scoop out some of the flesh, leaving a sturdy shell. Reserve the scooped-out potato for another recipe.
Meanwhile, fry the lardons or chopped pancetta in a frying pan until crispy. Drain off any excess fat.
Using a small, sharp knife, carefully carve simple skull features into the cut surface of each potato half. Make two eye sockets, a small nose and a jagged mouth with teeth.
Fill the carved openings with grated Cheddar and crispy bacon or pancetta.
Return the potato skins to the oven and bake for about 10–15 minutes, until the cheese has melted and the edges are crisp.
Scatter with chopped spring onions and serve warm with sour cream or crème fraîche on the side. For a dairy-free option, serve with tomato salsa or guacamole instead.