Preheat the oven to 300°F (150°C). Put the kettle on so you have hot water ready for the water bath.

Add the heavy cream, pumpkin puree and pumpkin pie spice to a small saucepan. Warm over low heat for about 5 minutes, whisking now and then, until it's steaming but not boiling. Stir in a few drops of orange food coloring if you're using it.

Meanwhile, whisk the egg yolks and the 2 tablespoons of sweetener together in a small bowl until smooth and pale.

Temper the eggs: whisk a spoonful of the warm cream mixture into the yolks, then another, so the yolks warm up gradually without scrambling. Pour the tempered yolk mixture back into the saucepan and whisk well to combine. Don't put the pan back on the heat.

Divide the custard between two 6-ounce ramekins (or three smaller 4-ounce ones) and set them in a small baking pan or dish. Pour hot water into the pan until it comes about halfway up the sides of the ramekins.

Bake for 30 to 35 minutes, until the edges are set but the centers still wobble slightly when you nudge the pan. Carefully lift the ramekins out of the water and let them cool on a rack.

Cover and refrigerate for at least 3 hours, or overnight, until completely chilled.

Just before serving, blot any moisture off the tops with a paper towel. Sprinkle 1 teaspoon of granular sweetener evenly over each custard. Melt it with a kitchen torch, keeping the flame moving about 2 inches above the surface, until it bubbles and turns glassy. (Or place the ramekins under a hot broiler for 1 to 3 minutes, watching closely.) Let them stand for 5 minutes so the top hardens, then serve.