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

In a heatproof bowl, combine the semisweet chocolate and unsalted butter over a double boiler, stirring until melted and smooth.

Remove from heat and whisk in granulated and light brown sugars. Add eggs one at a time and mix well, then add vanilla extract.

Sift flour, cocoa powder, and salt in a separate bowl. Gradually fold this into the wet ingredients until just combined.

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

Remove from the oven and let the brownies cool completely in the pan on a wire rack.

For ganache, heat heavy whipping cream until simmering, pour over chopped semisweet chocolate, and stir until smooth.

Lift the cooled brownies from the pan, cut into 1-inch squares, and dip each square into the ganache.

Allow excess ganache to drip off and place on a wire rack to set.

Before the ganache fully sets, sprinkle a pinch of fleur de sel on top of each petit four.