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

Melt the semisweet chocolate and butter together in a medium saucepan over low heat until smooth.

Stir in the granulated sugar and light brown sugar until fully combined. Add eggs one at a time while whisking.

Sift together flour, cocoa powder, and salt in a separate bowl. Gradually fold into the wet mixture until fully combined.

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

Allow the brownies to cool completely in the pan before transferring them onto a cutting board and cutting into small squares.

Heat heavy whipping cream over medium heat until it simmers. Pour it over chopped semisweet chocolate and let sit for two minutes before stirring until smooth.

Pour the ganache over each brownie square, allowing it to coat evenly while letting excess drip off.

Sprinkle fleur de sel lightly over each petit four to enhance the flavors beautifully.