Instructions: Preheat the oven to 350 F. Butter and flour a 10-inch tart pan with a removable bottom. In a medium saucepan over moderate heat, combine the butter and the sugars. Cook, stirring constantly, until the butter is melted, 4 to 6 minutes. In the bowl of a food processor, combine the bittersweet and unsweetened chocolate and process until finely chopped, 15 to 20 seconds. Add the hot butter-sugar mixture and process for 15 more seconds. Scrape the sides of the bowl and add the eggs and vanilla. Process to combine, about 10 seconds. Add the flour, salt, walnuts, and Grand Marnier. Pulse briefly 10 times, just to mix.

Spread the batter evenly into the prepared pan and sprinkle the top with chocolate chips. Bake 25 to 30 minutes; a toothpick inserted near the center will come out moist. Do not overbake. Remove the sides of the pan and cool on a wire rack. Cut into wedges and serve.

SERVES 6 TO 8.

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