1. Preheat the oven to 400 degrees F.

2. Rub the potatoes with about 1 tablespoon of the oil and season lightly with salt and pepper. Place the potatoes on a baking sheet and bake until the skins are slightly crisp and the inside is tender and can easily be pierced with a paring knife, about 45 minutes.

3. Meanwhile, put the bacon in a cold medium, skillet and cook over medium heat, turning occasionally, until crisp and brown, about 10 minutes. Drain briefly on paper towels then transfer to a cutting board and finely chop.

4. Halve the potatoes lengthwise. Scoop out most of the potato flesh, leaving about 1/4-inch of flesh on the skin. Arrange the potatoes, the baking sheet and brush both sides with butter. Season lightly with salt and pepper, arrange cut-side up, then return to the oven and bake until crisp and golden brown in spots, about 10 minutes.

5. Toss the cheese with the cornstarch in a bowl. Bring the beer to a simmer in a medium saucepan over medium heat. When it is simmering, add the cheese to the beer a handful at a time, stirring until it is melted before adding more. When all the cheese has melted, remove from the heat and stir in the Worcestershire, mustard, and cayenne.

6. Arrange the potato skins on a platter and drizzle with the beer cheese. Top with the bacon and scallion. Serve immediately.