Preheat oven to 350ºF; line an 8-inch square baking dish with parchment. Place butter in a small saucepan over medium-low heat. Cook, swirling the pan occasionally until butter is lightly browned in color and emits a nutty fragrance, watching carefully to prevent burning, about 3 minutes. Transfer to a large bowl to cool. In a small bowl, whisk together flour, baking powder, and fine salt. Whisk sweetener into butter, then whisk in egg and vanilla. Fold in flour mixture, then chocolate chips, until well incorporated. Spread batter in baking dish. Sprinkle with flaky salt, if using. Bake until golden and just set in the center, about 20 to 23 minutes. Let cool in pan on a wire rack before cutting into bars. Cover and refrigerate leftovers.