Preheat oven to 300℉. Arrange 4 - 6 creme brûlée dishes or ramekins in two 9 x 13 x 2-inch baking pans or a large roasting pan.
In a medium saucepan, combine heavy cream, sugar, nutmeg and salt. Bring to a simmer over medium-high, stirring to dissolve the sugar, about 2 - 4 minutes. Remove from heat and add the rum.
In a medium bowl, whisk the egg yolks. Add the warm cream mixture in a slow stream to the egg yolks, whisking constantly. Strain the custard through a fine-mesh sieve into a clean bowl or liquid measuring cup.
Divide custard between the dishes, filling each ¾-full. Carefully pour enough hot water into each baking pan to come halfway up the sides of the dishes. Cover pans with foil and bake for 20 minutes and the edges of the custard are set, but the center has a slight jiggle when the dish is lightly shaken.
Remove dishes from the water bath and let cool completely to room temperature. Refrigerate for at least 2 hours. Just before serving, sprinkle the top of each custard evenly with 1 ½ tablespoons of superfine sugar. Use a kitchen blowtorch to evenly melt and caramelize the sugar. Allow the caramelized sugar to cool and harden for approximately 5 minutes before serving.