6 ounces dark chocolate with 70 percent cocoa content, chopped;
¼ cup (2 ounces, ½ stick) unsalted butter, cut into 8 pieces;
2 tablespoons Grand Marnier or other good-quality orange-flavored liqueur;
1 tablespoon freshly grated orange zest (from 1 large orange);
1 teaspoon vanilla extract;
2 large eggs plus 1 egg yolk, at room temperature, lightly beaten;
1 partially baked Whole Wheat and Butter Tart Crust, plain or the citrus variation;
⅔ cup coarsely chopped toasted walnuts;
12 toasted walnut halves, for garnish.
Position a rack in the center of the oven and preheat to 350 F.
Warm the sugar and milk in a small heavy-bottomed saucepan over medium-low heat, stirring occasionally, until the sugar has dissolved, 4 to 5 minutes. Add the honey and cook until it has dissolved and the mixture is smooth, 1 minute or more, depending on the consistency of the honey. Set aside.
Place the chocolate and butter in a large metal bowl set over a saucepan containing about 1 inch of barely simmering water (the bottom of the bowl should not touch the water). Wait until
melted, stirring gently with a wooden spoon, about 4 minutes. Remove the bowl from the saucepan and set the chocolate mixture aside to cool for 5 minutes. Stir the Grand Marnier, orange zest, and vanilla extract into the sugar-honey mixture. Using a large whisk and a gentle hand, add the sugar-honey mixture to the chocolate mixture, and then whisk in the eggs and yolk just until incorporated. The mixture will thicken slightly.
After the crust has cooled, place the tart pan on a large rimless baking sheet for easier handling. Sprinkle the crust with the chopped walnuts. Gently spoon the filling evenly into the crust as to not disturb the nuts.
Carefully place the sheet with the tart pan in the oven (it can slide!) and bake for 15 minutes. Remove from the oven and decorate the tart with the walnut halves by lightly pressing them around the outer rim. Continue to bake until the filling barely jiggles when the pan is moved gently, 8 to 10 more minutes (it will puff around the edges, but settle as it cools). Remove from
the oven and carefully slide the tart pan onto a wire rack. Leave the tart to cool completely in the pan, about 1½ hours, before serving. The tart can sit at cool room temperature, under a cake
dome, up to 4 hours.
When ready to serve, remove the outer ring of the tart pan. Cut the tart into 12 pieces using a sharp knife dipped into hot water and wiped clean between each cut.
TO GET A HEAD START: The tart, including the filling, can be prepared 1 day ahead. Wait until the tart has completely cooled and then chill, covered with plastic wrap. Bring to room temperature before serving.