Directions

In a light-colored saucepan, melt the butter over medium-low heat, swirling the pan occasionally, until it starts to foam. Use a heatproof rubber spatula to scrape the bottom and sides of the pan occasionally to prevent the milk solids in the butter from burning. Cook for 5 minutes, or until the butter smells nutty and is amber with dark flecks at the bottom of the pan. Remove from the heat and immediately pour the brown butter into a liquid measuring cup. Set it on a wire rack to cool while you prep the other ingredients
Position a rack in the center of the oven and preheat the oven to 350º
Lightly spray a 9 x 13-inch cake pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on the pan's two long sides. Spray the parchment too
In a food processor, pulse the white chocolate a few seconds at a time until roughly chopped
Place the sugar in the bowl of a stand mixer fitted with the paddle attachment. With the mixer on low, slowly pour in the brown butter and beat for 1 minute, until the mixture looks like wet sand. Add the eggs one at a time, adding the next egg only after the previous one has been fully incorporated, using a rubber spatula to scrape down the bottom and sides of the bowl after each addition. Add the vanilla and beat until just combined. With the mixer on low, gradually add the dry ingredients and beat until just combined. Scrape down the bottom and sides of the bowl once more, and beat on low for an additional 30 seconds. Add the white chocolate all at once, and beat on medium-low until the chocolate is evenly distributed throughout, about 1 minute
Scrape the batter into the prepared pan and use an offset spatula to smooth the top. Bake for 25 to 30 minutes, or until a skewer inserted into the center of the blondies comes out with a few crumbs attached. Cool completely on a wire rack before slicing
Run a butter knife or offset spatula along the edges of the pan and use the overhanging parchment as handles to lift the blondies out of the pan and onto a cutting board. Slice into 20 rectangles, each about 2 1/4 inches long, and serve. The blondies can be stored in an airtight container or zip-top bag at room temperature for up to 3 days.