Preheat your oven to 350°F (175°C) and grease an 8x8 inch baking pan, lining it with parchment paper for easy removal. In a saucepan over low heat, melt the semisweet chocolate and unsalted butter together, stirring until smooth and glossy. Remove from heat, then mix in the granulated sugar and light brown sugar until fully combined. Beat in the eggs one at a time, whisking well after each addition, and stir in the vanilla extract for added flavor.

In a separate bowl, sift together the all-purpose flour, cocoa powder, and salt, ensuring there are no lumps. Gradually fold this dry mixture into the wet brownie batter using a rubber spatula. Mix just until incorporated to avoid overmixing; the batter should be thick and slightly lumpy.

Pour the brownie batter into the prepared pan, smoothing it out evenly with an offset spatula. Bake in the preheated oven for 20–25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs attached. Allow the brownies to cool completely in the pan on a wire rack before cutting to maintain their shape and texture.

While your brownies cool, prepare the ganache. In a small saucepan, heat the heavy whipping cream over medium heat until it begins to simmer, but do not let it boil. Remove from heat and pour the hot cream over the chopped semisweet chocolate in a bowl. Let it sit for 2-3 minutes to soften the chocolate, then stir until the mixture is smooth and glossy.

Once the brownies have cooled, use a sharp knife to cut them into small squares or rectangles, depending on your desired size. Dip each brownie piece into the silky chocolate ganache, ensuring it’s fully coated, and allow the excess to drip off back into the bowl. Place the coated brownies on a wire rack to set.

Before the ganache fully sets, sprinkle a pinch of fleur de sel over each brownie petit four. Allow the petit fours to rest on the wire racks until the ganache hardens.