Position rack in center of oven. Preheat to 350°F/180°C. Line the bottom of a 13 by 9-inch (33 cm by 23 cm) baking pan with parchment paper with the two short ends overhanging. Coat parchment paper with nonstick spray.

Whisk together the flour, cocoa and salt in a bowl to aerate and combine; set aside.

Beat the melted butter, sugar, brown sugar, eggs and vanilla in a large mixing bowl with an electric mixer until smooth, creamy and light in texture. The color will be light to medium beige. Beat in the melted bittersweet chocolate. Fold in the dry mixture until well incorporated.

Scrape batter into prepared pan and smooth the top with a small offset spatula. Scatter the white chocolate all over the surface and use your finger or a chopstick to poke some of it down into the batter, allowing about half of the white chocolate to remain on the surface.

Bake for about 18 to 22 minutes or just until a toothpick inserted into center comes out clean. Cool pan on rack for several hours. Use paper overhang to remove brownies from pan, place on cutting board, peel paper down and away and place brownies directly on cutting board. Cut into a 6 by 4 grid (24 brownies). Brownies are ready to eat and may be stored in an airtight container for up to 3 days (although they are best the day of and the day after).