Nigella's Creme Brulee suggests using a blowtorch to melt the sugar topping as it provides a concentrated blast of heat close to the sugar, causing it to caramelize quickly without causing the custard underneath to warm up. It is possible to use a very hot grill to do this, but you do need to make sure that the dish you have cooked the creme brulee in is able to withstand the direct heat of a grill. Preheat the grill for a good 5 minutes before trying to grill the sugar and put the dish as close to the heat source as possible. You may need to move the dish around to make sure the sugar is caramelizing evenly, as grills often have hot spots and cold spots. Some people like to stand the dish in a shallow pan of ice water while the sugar is being grilled, to help reduce the risk of the custard becoming warm and cooking further. Caster sugar can be used to top the creme brulee, though it can take longer for the sugar to brown and caramelize so a blowtorch is slightly better if you are using this substitute.