Preheat oven to 350°F (175°C). Spray an 8 x 8-inch pan with cooking spray, then line it with parchment paper, leaving an overhang for easy removal.
Melt butter in a light-colored, wide saucepan over medium-high heat. Once melted, reduce to medium and stir constantly with a silicone spatula. Butter will bubble and foam. When it smells nutty and you see light-brown specks under the foam, quickly remove from heat and pour butter into a bowl, scraping all the bits from the pan. Let it cool for 5 minutes.
Add the brown sugar to the cooled butter and whisk vigorously until the mixture resembles wet sand. Add the egg, vanilla, salt, and cinnamon, and whisk briskly until the mixture is very creamy and emulsified. Add the flour, chocolate chips, and nuts, then switch to a spatula to fold the dough until no dry streaks of flour remain. The batter will be thick.
Spread the batter evenly into the prepared pan and smooth the top with a spatula. Bake for 25–27 minutes, or until the edges are lightly golden and the top is set.
Remove from the oven and let the bars cool in the pan for 10–15 minutes. Use the parchment-paper overhang to transfer the bars to a cooling rack, then remove the parchment paper. Cut into bars and serve.
Notes
Note 1: Browning butter is much easier with a large, light-colored saucepan so you can see when the butter specks turn brown. This simple switch made all the difference for me!Storage: These bars are best enjoyed on the first day when they’re fresh and soft. Store leftovers in an airtight container at room temperature—they’ll still be good the next day, but not as soft. For longer storage, you can freeze them, but they’re best fresh.