1 1/4 cups all-purpose flour
1/4 cup granulated sugar
1/4 teaspoon salt
1/2 cup (1 stick) cold unsalted butter, cut into small pieces
3 tablespoons heavy cream
1 egg yolk
2 cups toasted walnuts, coarsely chopped
1/2 cup dark chocolate chips
3/4 cup packed light brown sugar
1/4 cup dark corn syrup
1/4 cup (1/2 stick) unsalted butter
2 tablespoons heavy cream
3 tablespoons coffee liqueur, such as Kahlua
Preheat the oven to 375 degrees F. To prepare the crust, pulse the flour, sugar and salt in a food processor until combined. Add the butter and pulse until coarse crumbs form. Whisk together the heavy cream and egg yolk in a small bowl. Slowly pour the heavy cream mixture into the feed tube and pulse until the dough begins to come together; form into a disk.
With lightly floured hands, press the dough evenly onto the bottom and up the sides of an 11-inch tart pan with a removable bottom, making sure to press the dough up to the rim. Bake until lightly browned, about 12 minutes. Transfer to a rack. Sprinkle the walnuts and chocolate chips evenly onto the bottom of the crust.
Meanwhile, to prepare the filling, combine the brown sugar, corn syrup, butter and cream in a medium saucepan over medium-high heat. Bring to a boil and cook, stirring constantly, until the sugar is completely dissolved, about 1 minute. Remove from the heat and stir in the coffee liqueur. Pour over the walnuts.
Bake until the mixture is bubbly around the edges, about 15 minutes. Cool on a rack. Serve warm or at room temperature. Tip: If the pie crust puffs up while baking, gently prick it in several places with a fork.