Butter a 13-by-9-inch baking pan and line with buttered parchment paper.

In top of a double boiler set over barely simmering water melt butter, chocolate and sugars together. Cool slightly.

Preheat oven to 350 F. In a large bowl or mixer, whisk eggs, salt and vanilla.

Whisk in chocolate mixture. Fold in flour just until combined. Stir in chopped walnuts.

Pour batter into prepared pan. Bake for 35 to 40 minutes or until shiny and beginning to crack on top. Cool in pan on rack.

For best flavor, bake 1 day before serving, let cool and store, tightly wrapped in plastic.