Preheat oven to 350°F (175°C). Line a 9×13-inch pan with parchment paper and lightly grease.

Melt butter and 8 oz chocolate together in a heatproof bowl until smooth.

Whisk in granulated and brown sugars until combined.

Add eggs one at a time, mixing well after each addition. Stir in vanilla.

In a separate bowl, whisk flour, cocoa, and salt. Gently fold into wet mixture until just combined.

Spread batter evenly in prepared pan. Bake 15–17 minutes or until a toothpick comes out with moist crumbs.

Cool completely, then freeze for 30 minutes before cutting.

For ganache: Heat cream until steaming, then pour over 8 oz chocolate. Let sit 1 minute, then stir until silky.

Cut brownies into rounds or squares. Place on a wire rack and pour ganache over tops.

Garnish with fleur de sel or sprinkles, if desired. Let set before serving.