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1
In a small saucepan over medium heat, melt the butter and cook until it turns brown and smells nutty, about 4 to 6 minutes. Stir frequently, scraping up any bits from the bottom so they don't burn. Remove from heat and immediately transfer brown butter to a liquid measuring cup (scraping all of the brown bits into the cup) and stir in water. Cool to room temperature (or 75u00b0F on an instant-read thermometer), either by refrigerating for about 30 minutes, or setting the measuring cup in an ice bath and stirring several times to speed up the cooling process. While it cools, proceed with Steps 2 and 3 and prep your ingredients.
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2
Center a rack in the oven and heat to 350u00b0F.
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3
Line a 9x13x2-inch aluminum baking pan with two overlapping sheets of aluminum foil, pushing the foil into the corners and up the sides and letting the excess overhang the pan edges. Grease lightly. (Alternatively, line the bottom of the pan with parchment if serving the cookie bars straight from the pan.)
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4
In the bowl of a stand mixer fitted with the whisk attachment, combine sugars, malted milk powder, salt, baking soda, vanilla, and eggs. Mix on low speed until just moistened, then increase speed to high and beat until thick and fluffy and several shades lighter in color, about 5 minutes (could be longer if using a hand mixer).
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5
Reduce speed to low, and drizzle in brown butter (making sure to scrape all of the brown bits into the bowl); mix until evenly incorporated. Add flour and cocoa; mix until just incorporated (do not overmix), then chocolate chunks and pecans. Transfer the batter into prepared pan, smoothing top with spatula.
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6
Bake until puffy, a few cracks have formed on the surface, the edges are starting to pull away from the pan, and the middle is just set, about 26 to 32 minutes, or an internal temperature of 205u00b0F in the middle (make sure the probe isn't touching the bottom of the pan). Cool on wire rack to room temperature. Remove bars from pan by lifting foil overhang and transfer to cutting board. Cut into 2-inch squares and serve.
