Heat oven to 350�. Line a 23 cm square baking pan with foil, letting ends extend above pan on 2 sides. melt chocolate and butter in microwave or in a medium saucepan over very low heat, stirring often, until smooth. Let cool 5 minutes.
With a whisk or electric mixer, add sugar and vanilla just until blended. Whisk in eggs, 1 at a time, just until blended. Whisk unitl mixture is velvety. Add flour and whisk until blended, then stir in nuts. Spread in pan.
Bake 35-45 minutes until top cracks near the edges and a tester inserted in the center comes out with moist crumbs. Cool completely on wire rack. Cut in 16 square, remove from foil..