Preheat oven to 350°F. Butter and flour a 13x9 inch baking pan.
In a medium bowl set over a pan of barely simmering water, melt the bittersweet and unsweetened chocolate with the butter. Stir until smooth and remove from heat. Cool to lukewarm.
Stir in the sugar and vanilla. Then add the eggs one at a time, stirring well after each. Add salt and flour and mix until just combined. Stir in the chocolate chips and chopped walnuts.
Pour batter into the prepared baking dish and smooth top. Bake until a toothpick inserted into the center comes out with moist crumbs, about 25-30 minutes. Cool completely in pan, then cut into bars.