Preheat the oven to 300 degrees F.

Prepare the caramel: In a heavy saucepan, combine the sugar and water. Cook over low heat, without stirring, until the syrup turns a medium amber, painting the sides of the pan with a wet pastry brush as needed, about 5 minutes. Pour into one large (10-cup) souffle dish or round casserole, and tilt to evenly coat the bottom and slightly up the sides. Set aside.

Prepare the custard: In the bowl of a food processor, process the potatoes. With the machine running, add 1 cup of the half-and-half, and process until smooth.

In a large bowl, combine the sugar, the remaining 1-cup of half-and-half, the condensed milk, 2/3 cup of water, the vanilla extract, and maple syrup, and mix well.

In another large bowl, beat the eggs and egg yolks until frothy. Strain the eggs into the wet ingredients and mix well. Add the sweet potato mixture, whisk well, then transfer to the prepared pan. Place the dish in a large baking pan and add enough hot water to come halfway up the sides of the dish. Bake for 2 hours, or until the custard is mostly set but the center is still slightly wobbly. Remove from the oven and transfer to a rack to cool.

Refrigerate until very cold, at least 6 hours, or overnight. To unmold, run a thin knife around the sides of the custard and invert onto a serving dish. Place Chocolate Spider's Web on top of flan, and present at tableside. Cut into wedges and serve as desired.

Place a 10-inch square of parchment paper or waxed paper on a baking sheet. With a black marker, draw a spider's web that will fit on top of the Sweet Potato Flan.

Place the chocolate chips in the top of a double boiler, or in a metal bowl set over a pot of simmering water, and melt, stirring until smooth. Furl a piece of parchment or waxed paper to make a tight cone. Transfer the melted chocolate to the cone and cut off the bottom tip. Pipe the chocolate onto the parchment paper, tracing the spider's web.

Place the baking sheet with the design in the refrigerator or freezer until firm.

To serve, gently peel back the paper and transfer the chocolate web to the top of the Sweet Potato Flan.