To the bowl of a stand mixer fitted with the paddle attachment (or a large bowl and whisk), add the brown sugar, sugar, eggs, egg yolk, vanilla and salt and mix for 2 to 3 minutes on medium until thick, lightened in color and well combined. Whisk in the cooled brown butter until fully combined. It will resist combining and try to separate; it's important you whisk until it is completely combined and you see no traces of the brown butter.
On low mix in the baking powder for 30 seconds and then add the flour, mixing until barely combined and the flour has just disappeared. Grab a spatula and give it one more mix around the bowl, scraping the sides and the bottom of the bowl to make sure there aren't any pockets of un-mixed flour. The blondie cookie dough will be thick. Take care not to over-mix the blonde dough.
Scoop the dough into the prepared pan and use the back of a spoon or an offset spatula to smooth the tops of the blondies (it doesn't have to be perfect). Sprinkle the chocolate chips over the tops and bake in the center of the oven for 25 to 30 minutes (27 minutes is perfect in my oven). The edges of the blondies should be golden brown, the top puffed up a bit and there will be some jiggle in the center of the pan. That's perfect. Sprinkle with sea salt flakes (if using) and set on a wire rack until completely cool.
See notes below for tips on cutting brownies. Wait until they are completely cool (which will take 2 to 3 hours) before cutting. You can additionally set them in the fridge to make cutting them even easier. Use a very sharp knife and clean the blade in between cuts for the sharpest blondies slices. Enjoy!