To prepare the crust, in a bowl, using an electric mixer on low speed, cream the butter and sugar together. Add the egg yolk, cream, and vanilla and mix well. In another bowl, sift together the flour, cocoa, and salt. Mix the flour mixture with the wet ingredients until incorporated. Press into a square on a piece of plastic wrap, wrap in the plastic, and refrigerate for 1 to 2 hours.
Remove the dough from the refrigerator, unwrap, and knead on a floured work surface until the dough is pliable. If using a 10-inch fluted tart pan with a removable bottom, roll the dough out to 1/4-inch thick and 12 inches in diameter. Spray the tart pan with vegetable-oil cooking spray. Carefully transfer the dough to the prepared pan and press firmly into the bottom and sides of the pan. Using a rolling pin, press down on the pan edges to trim off the excess dough. If using individual 3-inch tartlet pans with removable bottoms, divide the dough into 8 equal parts. Roll each one out to 1/4 inch thick and 4 inches in diameter. Spray the tartlet pans with vegetable-oil cooking spray. Carefully transfer each dough round to a prepared tartlet pan and press firmly into the bottom and sides of the pans. Using a rolling pin, press down on the edges of the pans to trim off the excess dough. Refrigerate the lined tart or tartlet pans for 20 minutes.
Preheat the oven to 350 degrees. Line the pastry shells with aluminum foil. Bake the 10-inch tart for 15 minutes, or the tartlets for 10 minutes, until the pastry begins to pull away from the sides of the pan. Remove the shells from the oven and allow to cool before removing the foil. Decrease the oven temperature to 300 degrees.
To prepare the filling, in a small saucepan, melt the butter over low heat. Stir in the chocolate until completely melted. In a bowl, mix the eggs, sugar, and corn syrup together. Add the chocolate mixture to the egg mixture and mix well. Add the vanilla extract and mix well. Fold in the nuts.
Pour the filling into the prebaked shells. Bake the 1-inch tart for about 1 hour and 20 minutes, or the tartlets for 35 to 40 minutes, untill the filling puffs up in the center but is not cracked. Allow to cool on racks and then refrigerate for 2 hours to make unmolding easier.
To prepare the whipped cream, combine all the ingredients in a bowl, and whip with an electric mixer on high speed until stiff peaks form.
To unmold, remove the pan sides and slide the pie or tarts onto a serving plate. Serve the pie at room temperature with the whipped cream and chopped nuts.
Makes 1 (10-inch) pie or 8 individual tarts.
Recipe courtesy of Beverly Gannon's The Hali'imaile General Store Cookbook (Ten Speed Press, 2000)