Preheat the oven to 200°C (400°F). Melt the butter in a small pot or in the microwave. Lightly coat each apple in the melted butter, then roll in the caster sugar until evenly covered. Arrange the apples upright in a small ovenproof roasting dish, leaving space between them.
Roast
Bake the apples for 25–30 minutes, or until lightly golden and tender.
Make the butterscotch sauce
While the apples roast, add the muscovado sugar, butter, cream, and vanilla to a small pot. Heat until bubbling, then reduce to a low simmer for 4–5 minutes, stirring occasionally, until the sauce begins to thicken. Remember, it will thicken further as it cools.
Finish and serve
Stir the hazelnuts into the sauce or sprinkle them over the apples when serving. Place the roasted apples on plates, drizzle generously with hot butterscotch sauce, and add a scoop of vanilla ice cream.
