Preheat your oven to 350°F (175°C).
Line a 9x9-inch baking pan with parchment paper.
In a mixing bowl, whisk together the melted unsalted butter and light brown sugar until smooth.
Add the egg and vanilla extract to the mixture and whisk until fully incorporated.
In another bowl, combine the all-purpose flour, baking powder, and fleur de sel, then gradually stir into the wet mixture.
Gently fold in the semisweet chocolate chips and toffee bits until evenly distributed.
Pour the batter into the prepared baking pan and spread evenly. Bake for 20-25 minutes.
Sprinkle a pinch of fleur de sel on top while still warm.
Allow to cool for 10-15 minutes, then lift from the pan and cool completely before slicing.
