Preheat the oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper.

In a medium saucepan, melt the semisweet chocolate and butter together over low heat. Stir until smooth.

Remove from heat and stir in granulated sugar and light brown sugar until fully combined.

Add in the eggs one at a time, whisking after each addition. Stir in the vanilla extract.

Sift together the flour, cocoa powder, and salt. Gradually fold the dry ingredients into the wet mixture until fully combined.

Pour the brownie batter into the prepared pan and smooth the top. Bake for 20–25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow the brownies to cool completely in the pan. Once cool, transfer to a cutting board and cut into small squares or rectangles to create petit fours.

For the ganache, heat the heavy whipping cream in a saucepan over medium heat until it begins to simmer.

Remove from heat and pour over the chopped semisweet chocolate. Let it sit for 2–3 minutes to melt, then stir until smooth and glossy.

Pour the ganache over each petit four, letting it coat evenly. Allow any excess to drip off and let the ganache set for a few minutes.

Sprinkle fleur de sel lightly over each petit four for a perfect salty-sweet balance. Let the ganache set completely before serving.