1. Preheat oven to 375 degrees. On a lightly floured surface, roll the dough 1/8 inch thick. Fit dough into 2 rectangular 13 3/4-by-4 1/4-inch tart shells, letting it fall naturally into the corners and being careful not to stretch the dough. With your fingers or a rolling pin, trim dough so that it is flush with top edge of tart shells. Prick the tart shell all over with the tines of a fork. Chill until firm, at least 45 minutes.

2. Line dough with parchment paper and fill with dried beans or lentils. Transfer to oven and bake until tart shells begin to color, about 20 minutes. Carefully remove the dried beans and parchment and return to the oven and bake until the tart shells become a deep golden color, about 15 to 20 minutes more. Cool completely on a cooling rack.

3. In a heavy saucepan, dissolve 1/2 cup sugar and the cornstarch in the cold milk. Stir in the scalded milk, place the mixture over direct heat, and boil for 3 minutes, stirring constantly. Transfer the mixture to the top of a double boiler.

4. Beat the egg yolks in a small bowl and stir in 3 tablespoons of the milk mixture. (This will keep the egg yolks from curdling when added to the remaining hot milk.) Pour this back into the remaining milk mixture, and cook over simmering water for 3 to 4 minutes, stirring constantly, until the mixture becomes quite thick. Remove from the heat and put 1 cup custard in a small bowl. Stir in the melted chocolate and vanilla, and set aside to cool completely. When cooled, spread evenly in the tart shell and chill until set.

5. Meanwhile, soften the gelatin in the cold water and add to the remaining hot custard. (The gelatin will not dissolve properly if the custard has cooled.) Stir to combine. Fold in the rum and let the mixture cool at room temperature until it just begins to set.

6. Beat the egg whites with the salt and cream of tartar until stiff but not dry. Gradually beat in the remaining 1/4 cup sugar, and then fold the egg whites into the custard mixture. Spoon this evenly over the chocolate mixture. Chill until set.

7. Remove the baking sheet from the refrigerator. Using a sharp metal pastry scraper held at a 45-degree angle, scrape off a strip of chocolate from the pan; the chocolate will curl as it is scraped. Chill or freeze until the curls are firm and ready to use.

8. The type of curl you get will be determined largely by the temperature of the chocolate when you remove it from the refrigerator. If it is very cold, the chocolate will splinter when scraped and give you small "thatch." If the chocolate is a little less cold, you will produce tight curls. And if the chilled chocolate approaches room temperature (or if a lightly pressed finger leaves a slight print), you will have big, loose curls or sheets of chocolate, which can be used as "bark." The type of curl you produce will also depend on how you scrape the chocolate from the sheet.

9. Whip the cream until stiff and spread or pipe over the custard. Garnish as desired with chocolate thatch. Chill until ready to serve.