6 tablespoons unsalted butter, diced
3 ounces unsweetened chocolate, chopped
1 1/4 cups packed dark brown sugar (about 9 ounces)
3 eggs, at room temperature
3/4 cup flour
1/4 teaspoon fine salt
1/2 cup walnut pieces
Walnut pieces (extra for top)
Chocolate pie crust
Preheat the oven to 350 degrees F.
Whisk the butter and the chocolate over low heat in a heavy medium saucepan until the chocolate is almost melted. Remove the pan from the heat and whisk until the chocolate is fully melted and the mixture is smooth. Whisk in the sugar, then the eggs, one at a time. Stir in the flour and salt with a flexible rubber spatula. Stir in the nut pieces. Scrape the batter into the chocolate pie crust. Top with extra walnut pieces.
Place the pie on a baking sheet and bake until the outer 2 to 3 inches are slightly puffed and dry looking and the center is set, 35 minutes. A wooden skewer inserted into the center should come out with some moist batter still attached. Cool the pie completely on a rack.
Cut the pie into wedges and serve with ice cream or whipped cream.