Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking pan by lining it with parchment paper.

In a heatproof bowl over simmering water, melt the semisweet chocolate and unsalted butter until smooth.

Remove from heat, whisk in granulated and light brown sugars, then add eggs one at a time, whisking vigorously.

Stir in the vanilla extract, then sift in flour, cocoa powder, and salt, mixing gently to combine.

Pour the batter into the prepared pan and bake for 20-25 minutes until a toothpick comes out with a few moist crumbs.

Cool in the pan for 10 minutes, then lift out and transfer to a wire rack to cool completely.

Heat heavy whipping cream until simmering, pour over chopped chocolate, and stir until smooth to make ganache.

Cut cooled brownies into small squares or rectangles, dip each piece into ganache, and place on a wire rack.

Sprinkle fleur de sel over the ganache-coated bites before setting, then allow to set for about 30 minutes.