Preheat the oven to 325 degrees F. and set the pie shell on a baking sheet.
In a saucepan, melt the butter over medium heat. Reduce heat to low and add chocolate. Stir until chocolate is melted. Let cool slightly.
In a mixing bowl, using an electric mixer, beat the eggs and egg yolk for about 2 minutes. Gradually add sugar and continue beating for another minute or so. Beat in the salt and vanilla, then scrape in the melted chocolate. By hand, stir in the flour.
Sprinkle the toasted walnuts across bottom of your pie crust, then pour filling over the nuts. If you used a 9 inch deep dish pie shell, you’ll have a little more filling than you need, so pour just enough filling into the pie crust so that it comes almost to the top rim.
Bake for 40-45 minutes or until the pie appears set and a knife inserted in the center comes out clean. Let cool completely.
Serve the pie with ice cream and chocolate syrup.