Step 1

Cut 2 sticks unsalted butter into large pieces. Melt in a small saucepan over medium-high heat, then continue cooking, swirling the pan occasionally — at first, it will simmer and look foamy, then the foam will subside and the solids at the bottom will begin to brown, 10 to 12 minutes total.

Step 2

Scrape the browned butter and the solids at the bottom into a stand mixer fitted with the paddle attachment. (Alternatively, transfer into a large heatproof bowl and use an electric hand mixer). Let cool to room temperature, about 30 minutes. If you’re using any add-ins that require chopping (like chocolate bars, pretzels, or nuts), now is a great time to chop them.

Step 3

Arrange a rack in the middle of the oven and heat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper so the parchment hangs off the two long slides to form a sling.

Step 4

Add 1 1/2 cups packed dark brown sugar and 1/2 cup granulated sugar to the butter. Beat on low speed until just incorporated. Add 2 large eggs and 2 teaspoons vanilla extract and beat on medium speed until the mixture is lightened in color and fluffy, about 3 minutes.

Step 5

Add 1 3/4 cups all-purpose flour, 2 teaspoons baking powder, and 1 teaspoon kosher salt and beat at low speed for 1 minute. Add 2 cups mix-ins to the batter and fold in by hand with a large, wide spatula.

Step 6

Scrape the thick batter into the prepared pan and spread into an even layer. Bake until the edges are browned and the top is crackled and shiny, 22 to 25 minutes.

Step 7

Cool the blondies in the pan for 10 minutes. Then just slip the slab of blondies out of the baking pan using the parchment paper sling, cut them on the parchment, and return the cut blondies, parchment and all, to the pan for transporting.