(Makes one tart, serving 8 generously. My tart pan with removable bottom measures 9½" across by 1¼" deep. I have also made this in a 9" pie plate, substituting a partially blind baked pie shell for the crumb crust. Equally delicious.)
A note about the crust:
If you happen to have an abundance of slightly crispy chocolate chip cookies on hand, process them in the bowl of your food processor fitted with the metal blade until they are finely ground. You will need 2 cups of crumbs for the 9½” tart pan. You can use any cookie crumbs for this; preferably something with a bit more texture than a plain graham cracker crust. If you don’t have cookies available, you can prepare a single recipe of pate brisée, line a 9” pie plate with it, chill the dough until firm, then partially blind bake it. Line the pie dough with a sheet of parchment paper or aluminum foil. Fill with pie weights or beans, and bake in a pre-heated 375 degree oven for about 18 minutes, until the dough is set. Remove parchment or foil and weights, return pie shell to oven for an additional 8 minutes, just until the pie shell is lightly browned. Set the pre-baked shell aside while you prepare the filling.
2 cups finely ground chocolate chip cookies
2 Tablespoons unsalted butter, melted
Pre-heat your oven to 350 degrees. Generously butter a 9½” tart pan with removable bottom. In a medium mixing bowl, stir together the cookie crumbs and the melted butter. Press the crumb mixture evenly into the bottom and along the sides of the tart pan. Chill for 15 minutes. Set the chilled tart pan on a baking sheet and bake for approximately 12 minutes. Let the tart shell cool while preparing the filling.
Chocolate Walnut filling (enough to fill a 9” pre-baked pie shell or a 9½” pre-baked tart shell)
2 Tablespoons all-purpose flour
1/3 cup packed dark brown sugar
¼ teaspoon kosher salt
½ cup dark corn syrup
2 Tablespoons unsalted butter
4 ounces bittersweet chocolate, coarsely chopped
½ teaspoon good quality vanilla extract
3 large eggs
2 cups walnut halves and pieces (you can find these at Trader Joe’s)
Pre-heat your oven to 350 degrees. In a heavy bottomed saucepan, whisk together, flour, brown sugar and salt. Add corn syrup and cook over medium heat until the sugar dissolves and the mixture comes to a boil. Continue cooking for another minute, remove from the heat and add the butter and chocolate. Stir with a rubber spatula until the chocolate and butter has melted and the mixture is smooth. Set aside until the mixture is cool to the touch.
In a large mixing bowl, whisk together the eggs and vanilla. Gradually add a few tablespoons of the chocolate mixture to the eggs, whisking constantly. Continue gradually adding the chocolate mixture to the eggs, whisking well so there are no streaks of egg visible.
Place the pre-baked tart shell on a parchment lined baking sheet. Sprinkle the walnuts over the bottom of the tart shell then slowly pour the chocolate filling over the walnuts. Smooth the top with an offset spatula, and place the baking sheet on the middle rack of the pre-heated 350 degree oven. Bake for 30 minutes, then check to see if the tart has puffed around the edges and the center feels firm. Test the center of the tart with a small paring knife, it should come out clean. If it is still wet, continue baking for an additional 5 minutes until it tests clean. Cool on a rack for at least 2 hours before serving. This is most delicious when it is slightly warm. (If there are any leftovers, cover with plastic wrap and refrigerate.)