Preheat oven to 350 degrees F. Grease an 8-inch square baking pan and line with parchment paper.
Place the butter and chocolate in a medium heatproof bowl set on top of a small saucepan of barely simmering water and stir occasionally until completely melted and smooth. Remove the bowl and set aside to cool slightly.
In a large bowl, whisk together the granulated sugar, dark brown sugar, eggs, and vanilla extract until pale and fluffy. Using a rubber spatula, stir in the melted chocolate mixture to combine. Sift the flour and cocoa powder into the chocolate mixture, add the fleur de sel and fold until no streaks of flour remain. Gently fold in the chopped Rolo candies.
Pour the batter into the prepared pan and smooth into an even layer. Press the whole Rolo candies into the surface of the brownie batter, evenly spaced. Bake until the surface is set and a toothpick inserted in the center comes out with moist crumbs attached, about 25 to 30 minutes. Cool completely in the pan, then transfer to a cutting board and slice. Leftover brownies should be stored in an airtight container at room temperature for up to 5 days.