Preheat oven to 350°.
Grease the bottom only of a 13X9 baking dish, or line with parchment paper.
In a separate bowl combine flour, cocoa and salt, whisk together and set aside.
Break up the bittersweet chocolate into small squares.
Melt butter and the bittersweet chocolate in the microwave at 20 second intervals, stirring after each 20 seconds, until smooth.
Cool to room temperature.
Add the sugar and vanilla stir until mixed.
Whisk in the eggs one at a time.
Combine flour mixture with the chocolate mixture, stir until incorporated.
Add chocolate chips and nuts.
Pour into prepared baking dish.
Bake for 30-35 minutes.
Cool in the baking dish on a wire rack.
Cool completely before cutting.