In saucepan over medium-high heat, bring sugar, butter and 2 tbsp water to boil, stirring occasionally. Remove from heat; stir in half of the bittersweet chocolate until melted. Let cool for 10 minutes. Whisk in eggs, 1 at a time; whisk in vanilla.
In separate bowl, whisk together flour, baking soda and salt; stir into chocolate mixture. Stir in remaining bittersweet chocolate and white chocolate. Pour into parchment paper-lined 9-inch square metal cake pan.
Bake in centre of 325F oven for 30 minutes or until tester comes out with a few crumbs clinging. Let cool in pan on rack.