Preheat oven to 350F and place rack in middle of oven.

Butter and flour a 13 x 9 inch baking pan.

Melt chocolate with butter, stirring, until mixture is smooth. Let cool until lukewarm.

Stir in the sugar, vanilla and eggs, one at a time, stirring well after each addition. Stir in the salt and flour, stirring just until combined. Stir in the chocolate chips and walnuts.

Pour into prepared baking pan and bake for 25-30 minutes, or until a tester comes out with a few crumbs attached.

Let cool completely and cut into 24 bars.