Butter a 13-by-9-inch baking pan and line with buttered parchment paper.
Preheat oven to 350 degrees.
In microwave, melt butter, chocolate and sugars together.
Cool slightly.
In a large bowl whisk eggs. Whisk in salt and vanilla.
Whisk in the chocolate mixture.
Fold in flour just until combined.
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.