Preheat your oven to 350°F (175°C).
Line a 9x9-inch baking pan with parchment paper, leaving overhang on the sides.
In a mixing bowl, whisk together the melted butter and brown sugar until smooth. Add the egg and vanilla extract, mixing until well combined.
In a separate bowl, whisk together the flour, baking powder, and fleur de sel.
Gradually fold the dry ingredients into the wet mixture using a spatula.
Gently fold in the chocolate chips and toffee bits until evenly distributed.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 20-25 minutes.
Remove from the oven, sprinkle with a pinch of fleur de sel, and allow to cool before cutting into squares.