Melt 1 cup chocolate chips with butter in a medium saucepan set over medium-low heat. Remove from heat; cool for 5 minutes.
Whisk in sugar and vanilla. Whisk in eggs, one at a time, until fully incorporated; add salt. Stir in flour until no dry streaks remain. Stir in remaining chocolate chips and walnuts (if using).
Scrape batter into pie shell and smooth top. Bake, on lowest rack, for 35 minutes or until a toothpick inserted into the centre comes out slightly moist. Serve warm with whipped cream.