Heat the oven to 350°F and set out a 9x13-inch glass baking dish.

Cut each frozen sweet potato patty in half with a sharp knife, straight from the freezer. Frozen patties cut cleanly; thawed ones smear and fall apart. You need 16 halves.

Unroll the crescent dough from both tubes and separate it along the perforations into 16 triangles. Set a sweet potato half at the wide end of each triangle, roll toward the point and tuck the sides in. Place each dumpling seam side down in the dish. They don't need to be sealed perfectly.

In a medium saucepan over medium heat, melt the butter until it bubbles. Add the sugar, cinnamon, nutmeg and vanilla and stir until the sugar is mostly dissolved.

In a small bowl, whisk the cornstarch into the cold water until no lumps are left. Stir it into the butter mixture until smooth.

Pour the sauce evenly over the dumplings. It will look like far too much liquid; it reduces and soaks in as the dumplings bake.

Bake for 35 to 40 minutes, until the dumplings are golden brown and some of the liquid has been absorbed. Start checking at 35 minutes, because the sugar can go from golden to too dark in about 4 minutes.

If the sauce still looks thin, let the dish rest on the counter for 10 to 15 minutes; it thickens as it cools. Serve warm from the dish with the sauce spooned over, and a scoop of vanilla ice cream if you like.