Preheat oven to 350 degrees F. Line an 8-inch square baking dish with parchment paper.
In a large bowl, whisk together the melted butter and brown sugar until completely smooth and there are no lumps. Add the egg and whisk until it is thoroughly combined. Add the vanilla extract and salt and whisk to incorporate.
Using a rubber spatula, stir in the flour and baking powder until most of the dry ingredients have been absorbed, then add the chocolate chips and toffee bits. Stir until there are no streaks of flour remaining.
Turn the batter out into the prepared pan and spread into an even layer (the batter will be thick, like cookie dough). Sprinkle with a pinch of fleur de sel. Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs attached. Allow the blondies to cool completely before cutting and serving. They can be stored in an airtight container at room temperature for up to 4 days.
Notes
Note: This recipe can be doubled and baked in a 9x13-inch baking dish.Nutritional values are based on one serving