Preheat the oven to 400 degrees F. Remove all wrapping and break apart the frozen pastry pieces along the pre-scored lines to separate the pastry cups. Place the pastry on a baking sheet (perforation-side-up). Bake 20 minutes, or until golden brown and puffed.
Make the pumpkin custard by combining the sugar, cornstarch, and salt in a small pan. Gently mix.
Add in the beaten egg yolk and heavy cream. Cook over medium heat, stirring constantly until smooth and thick. Add the vanilla, butter, cinnamon, and the pumpkin. Stir and set aside to cool.
When the pastries come out of the oven, use the end of a wooden spoon to press down the center of each pastry into bottom of cups.
Spoon the pumpkin custard into each pastry cup, not overfilling the cup.
Fill a small Ziploc baggie with whipped cream (or Cool Whip) and snip a very small piece of the corner of the bag with scissors.
Gently squeeze whipped cream onto the top of each custard-filled tart, swirling in a design if desired.
Sprinkle with fresh nutmeg. Serve immediately (while custard and cups are still warm), or serve them chilled.