Preheat oven to 350°F (175°C). Spray an 8x8-inch pan with cooking spray and line it with parchment paper, leaving an overhang for easy removal.
Melt the butter in a light-colored, large pan over medium-high heat. Once melted, reduce to medium heat and stir constantly with a silicone spatula. When the butter smells nutty and light-brown specks appear under the foam, remove it from the heat and pour it into a bowl, scraping out all the bits. Let it cool for 5 minutes.
Whisk brown sugar into the cooled butter until it resembles wet sand. Add the egg, vanilla, salt, and cinnamon, and whisk briskly until the mixture is creamy and emulsified. Fold in flour and white chocolate chips with a spatula until no dry streaks remain. The batter will be thick.
Pour the batter into the prepared pan and smooth the top with a spatula. In a small bowl, stir together the granulated sugar and ground cinnamon for the topping. Sprinkle this mixture evenly over the batter.
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.
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Notes
Note 1: Browning butter is much easier with a large, light-colored pan 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 any 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 at their best when eaten fresh.