Melt chocolate and butter in a double boiler over hot, but not boiling, water, stirring often. Alternatively, melt chocolate and butter in a large microwave safe bowl in the microwave at 600 watts for 30 second intervals, stirring in between, until chocolate and butter are smooth. Cool until barely warm. While the chocolate mixture is cooling, preheat oven to 400°F (190°C) and butter a 9x13 inch (24x40 cm) baking pan.

Stir sugar, eggs and vanilla into chocolate mixture. Beat lightly with a rubber spatula until combined. Fold in flour, salt and nuts.

Spread batter in prepared pan and bake for 16-18 minutes, until a toothpick inserted in center comes out with moist crumbs. I bake mine for exactly 16 minutes. Cool on wire rack. Store tightly covered in refrigerator for up to one week or freeze for up to three months.