Special equipment: 2 disposable 5-inch pie tins

For the crust: Put a small baking sheet on a rack in the center of the oven, and preheat to 375 degrees F. Process the flour, butter, granulated and confectioners' sugars and salt in a food processor until the mixture resembles wet crumbs and holds together when squeezed in your palm, about 2 minutes (this takes longer than seems necessary, but be patient). Divide the mixture evenly between 2 disposable 5-inch pie tins, and press the dough very firmly into the bottoms and up the sides, making sure to press it into the corners very well. Smooth and flatten the tops with your fingers or the back of a small spoon.

Bake the crusts on the hot baking sheet until the bottom and sides are golden brown (probably more in some spots than in others), about 12 minutes. Let cool completely on a cooling rack.

For the filling: Whisk together the eggs, cream, brown sugar, vanilla, cinnamon and allspice in a medium bowl. Whisk in the sweet potato and then the melted butter.

Mound the filling in the pie shells, and bake until puffed and the centers still look slightly wet but not loose, 25 to 30 minutes. Transfer to a cooling rack to cool completely before serving.

If using marshmallows: Right before serving, preheat the broiler. Pile 1/4 cup mini marshmallows in the center of each pie, and broil until the marshmallows are bubbling and well browned, 30 to 40 seconds. Serve immediately.

If not using marshmallows: Dollop each pie with some whipped cream, and serve.