To prepare the tart shell: In a food processor, combine flour, sugar, and salt; whirl briefly to blend. Add lemon zest and butter; pulse machine on and off until mixture resembles coarse meal. With machine on, add water, a few drops at a time, just until dough begins to form a ball. Flatten dough into a 6-inch disk, wrap in plastic wrap, and refrigerate until firm, about 30 minutes.
Lightly coat a 10-inch diameter tart pan with a removable bottom with vegetable oil spray. With you fingertips, press dough evenly onto the bottom and up the sides of the pan. Freeze until firm, about 30 minutes. Preheat oven 400°.
With a fork, prick bottom and sides of tart shell all over. Fit a 12-inch square of parchment paper (or a double thickness of aluminum foil with a few holes poked in it) snugly against pastry in pan. Bake until edges of pastry are light golden brown, 10 to 15 minutes. Carefully remove parchment, reduce oven temperature to 375°F. Bake until tart shell is golden brown all over, 15- 20 minutes. Let cool on a wire rack.
To assemble and bake the tart: Preheat oven to 350°. Spread walnuts on a baking sheet or in a shallow pan. Bake, stirring once or twice, until lightly browned and fragrant, 10 to 12 minutes. Let cool. Leave oven set at 350°F.
In a large, heatproof bowl fitted snugly over a saucepan containing 2 inches of hot water, melt chocolate and butter, stirring until smooth. Remove from heat.
In a medium saucepan, combine corn syrup and granulated sugar. Cook over medium-high heat, stirring, until mixture boils. Stir into chocolate mixture.
In a medium bowl, whisk eggs with brandy until foamy; stir into chocolate mixture. Add walnuts. Scrape mixture into prepared tart shell. Bake until filling is set and a toothpick inserted into the center comes out clean, 35-40 minutes. Let cool on a wire rack until just warm 15-20 minutes.
Meanwhile, in a large bowl, combine cream, 2 tablespoons of the powdered sugar, and vanilla. Whip until soft peaks form.
Dust tart with the remaining 1 tablespoon powdered sugar and garnish with Diamond walnut halves. Serve with the sweetened whipped cream.