Heat your oven to 325 degrees F. Grease a 9-inch square baking pan with nonstick cooking spray and line it with a sling of parchment paper.
Melt the butter in a small pot over medium heat. If desired, you can brown the butter by simmering the butter, stirring frequently, until golden brown flecks appear, 5 to 7 minutes. Whether using melted butter or brown butter, immediately transfer it to a large bowl and set aside to cool slightly.
Meanwhile, whisk together the flour, cornstarch, baking powder and kosher salt and set aside.
When the butter has cooled slightly, beat in the brown sugar and granulated sugar until well combined. Beat in the eggs, one at a time, and stir in the vanilla. Add the dry ingredients into the butter mixture and mix until almost combined. Scatter in most of the mix-ins and stir just until evenly distributed.
Spread the batter into the prepared baking pan and scatter over the remaining mix-ins as well as a pinch of flaky sea salt. Bake until the edges are golden brown and the middle is set, 30 to 35 minutes. Allow the cookie bars to cool slightly in the pan before lifting out and slicing into 12 equal bars. The cookie bars will keep covered on the counter for up to 5 days.
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