Brown the butter about an hour before making the dough. You can find an entire step-by-step guide on how to brown butter here.
When ready to bake, preheat the oven to 325 degrees. Lightly coat a 9x13 light metal pan with baking spray, place a large piece of parchment paper longwise across the pan so that a few inches of paper hang over the edges, like handles. You can secure the edges of the paper with metal clips, if desired.
In a medium bowl, whisk together the flour, baking powder, 1 teaspoon of the flaky salt, and kosher salt and set aside.
In a large bowl, combine the dark brown sugar and eggs with a hand mixer on medium until smooth. Add the vanilla and sourdough starter, mixing again to combine. Scrape down the bowl before adding the cooled browned butter. Mix until thick and glossy, about a minute more.
Add the dry ingredients and 2 cups of the chocolate chunks to the wet ingredients. Stir with a study wooden spoon or rubber spatula, until just combined and no dry streaks of flour remain.
Spread batter into prepared pan and sprinkle with remaining chocolate chunks. Bake in preheated oven for 40 – 45 minutes, or until edges are risen and begin to pull away from the pan and the top of the blondies are deeply golden, shiny, and krinkly.
Remove from oven and immediately sprinkle with a generous dose of more flaky sea salt, if desired.
Cool bars completely on wire rack before slicing and serving.