Creamy, smooth ricotta is crucial here. Find it in cheese shops, speciality food stores, Italian markets, or make your own. The grainy ricotta sold in most supermarkets ruins the cake's melting texture. Do use fresh chestnuts if they are available. If not, the best prepared ones I have found are imported from France, roasted, and packed in jars without liquid (Minerve brand). Present the cake at the end of celebration dinners, and take it along to parties. It can be baked well in advance and travels well.
Roasting Chestnuts: Preheat oven to 400 degrees. Cut a slit two-thirds the way around each nut. Spread in a roasting pan. Bake about 45 minutes to an hour, or until tender when tasted. Let them cool about 10 minutes. Slip off the shells and the inner skin just under the shell.
Flavoring the Chestnuts: Combine the peeled chestnuts, vanilla bean, rum and the 1/2 cup of sugar in a 4-quart saucepan over high heat. Cook at a lively bubble 5 to 8 minutes, stirring often with a wooden spatula. Add a little water if the chestnuts threaten to scorch. Cook to very thick. The chestnuts should be almost whole. Cool the mixture and remove the vanilla bean. Crush chestnuts against side of the pan until broken into bite-sized pieces.
Finishing and Baking: Use the tablespoon of butter to grease the bottom and sides of a 10 1/2-inch springform pan. Preheat oven to 350 degrees. In a medium bowl, stir the ricotta, eggs, sugar, vanilla, and citron until well blended. Lace the chestnuts through the ricotta mixture, creating streaks like a marble cake. Do not overmix. Turn batter into the springform, smoothing the top. Bake in the center of the oven for 1 hour. Turn heat to 325 degrees, and bake another 15 minutes, or until a knife inserted about 2 inches from the edge comes out clean. The center of the cake will still be a little creamy. Remove cake from the oven and let cool.
Serving: Unmold the cake by releasing sides of the springform pan. Serve warm, or cool, but not cold. To serve cool, bring down to room temperature, wrap, and refrigerate. Let slices sit at room temperature about 15 minutes before serving.
Wine Suggestions: Typically wine is not taken with the cake, but a fresh fruity white Malvasia or Moscato are both lovely with it.
Adam Rapoport, editor in chief of Bon Appetit magazine and the website www.bonappetit.com, knows his way around a grill. He has edited an entire book on the subject: The Grilling Book: The Definitive Guide from Bon Appetit.