Pierce the squash all over with a fork and place on a damp paper towel in a shallow microwavable dish. Microwave on high until tender, about 15 minutes.
Let the squash cool, then cut in half horizontally and, using a spoon, scoop out and discard the seeds and tough fibers. Scoop out the flesh and set aside.
Preheat the oven to 325°F. Combine the eggs, egg yolks and cream in a food processor or blender. Add the cooked squash, sugar and nutmeg and blend until smooth.
Butter four 8-ounce ovenproof mugs or ramekins and place them in a 9-x-12-inch baking pan. Fill the mugs with the custard, then carefully pour hot water into the pan to reach halfway up the sides of the mugs.
Bake the custards until set, 30 to 40 minutes. Serves 4.