1 frozen puff pastry (14 oz), thawed
For the sweet potato filling:
2 medium sweet potatoes
2 Tbsp unsalted butter
2 Tbsp maple syrup
For the chocolate glaze:
8 oz 61% or 72% chocolate, chopped
1 Tbsp light corn syrup
1 cup heavy cream
For the optional garnish:
Fleur de sel
For the base: Place a rack in the center of the oven and preheat to 375°F.
Line a baking sheet with parchment paper. One a lightly floured surface, roll the puff pastry into a rectangle about 1/4-inch thick and at least 9 inches wide. Pierce the entire surface with a fork and brush off the excess flour. Place the puff pastry in the refrigerator for 1 hour.
Remove the puff pastry from the refrigerator. Using a round 9-inch cake pan as a guide, cut a 9-inch round out of the puff pastry. Remove the scraps.
Place a silicone baking mat or piece of parchment paper over the puff pastry, then cover with a wire cooling rack to keep it from rising too much. Bake for 15 minutes, then remove the rack and silicone baking mat and increase the heat to 450°F. Sift some confectioners' sugar over the top of the puff pastry, and bake for 5 more minutes, until the sugar begins to caramelize and darken. Remove the base from the oven, and set it aside to cool.
For the filling: Place a rack in the center of the over and preheat to 400°F. Rinse, peel and cut the sweet potatoes in half. Place them on a large piece of aluminum foil. Put 1/2 tablespoon of butter and 1/2 tablespoon of maple syrup over each half, and wrap them in the foil. Bake for about 1 hour, or until a fork easily pierces their flesh.
When the potatoes are cooked, remove them from the oven and immediately purée them in a food processor until smooth or pass them through a potato ricer.
Line a baking sheet with parchment paper, and place a 9 x 2 1/2-inch cake ring mold on the baking sheet. Fill the ring to the rim with the purée. Smooth the top of the ring with a large offset spatula, and place the baking sheet in the freezer until the filling sets, at least 1 hour or up to 1 week (wrapped tightly in plastic wrap).
For the chocolate glaze: Place the chocolate and corn syrup in a medium bowl.
Bring the heavy cream to a boil in a small saucepan over medium-high heat. Pour the hot cream over the chocolate and whisk until the chocolate is melted and the mixture is smooth. Pour over the tart and spread immediately.
To finish the tart: Remove the baking sheet from the freezer 1 hour before serving, so that the sweet potato purée has time to thaw out. Gently lift the ring from the filling. Place a wire cooling rack on the baking sheet, and place the frozen purée on it.
Pour the glaze over the purée, completely covering it. Push the glaze down the sides with an offset spatula if needed. When the glaze has set slightly, after 3 to 5 minutes, place the purée on top of the puff pastry.