Preheat oven to 350ºF.
1) In a 1-quart saucepan, heat heavey cream to just before boiling.
2) In a bowl, whisk together the sugar and the egg yolks to dissolve the sugar. Add the heavy cream slowly; mix well. Stir in the vanilla extract and optional flavoring.
3) Strain the custard and pour into four 2-inch high ramekins.
4) Place the ramekins in a large baking pan. Pour enough hot water into the pan to come half way up the sides of the ramekins.
5) Bake until firm in the center, about 30 minutes. Remove the ramekins from the water bath and let cool completely.
6) Place in the refrigerator for 1 to 2 hours.
7) To serve, evenly sprinkle the demerara sugar over the custards and use a blow torch to caramelize the tops. You can also use the broiler to caramelize the sugar.