Preheat oven to 350° F (180° C) and grease a 13- by 9- inch (3.5 L) cake pan
In a large saucepan over low heat, melt chocolate and butter stirring until smooth. Set aside to cool slightly.
When chocolate mixture has cooled, stir in sugar. Add eggs and vanilla and beat until just combined. Blend in flour. Stir in nuts.
Spread batter evenly in prepared pan. Bake in preheated oven for 30 to 35 minutes or until a tester inserted in the center comes out clean. Place pan on a rack to cool completely. If desired, frost, then cut into squares.
Notes
Instead of chopped nuts I went with sunflower seeds. It was a delicious revelation.