Prepare the pan. Preheat an oven to 350°F (175°C). Line an 8x8 baking pan with parchment paper, leaving a few inches overhang on two sides, then grease with butter or nonstick baking spray.
Make browned butter. Place 12 Tbsp (170g) unsalted butter in a medium saucepan, then cook over medium heat, stirring occasionally, until the butter turns foamy and is a deep golden brown color, about 3-5 minutes. Cool for 5 minutes.
Mix the wet ingredients. In a large bowl, whisk together 1 cup (200g) brown sugar, 3 Tbsp (60g) maple syrup, and 2 large (100g) eggs until very well mixed, about 1-2 minutes. Pour in the melted browned butter, then mix again.
Add the dry ingredients. Add 1⅔ cup (200g) all-purpose flour, ½ tsp Kosher salt, ½ tsp baking powder, then use a spatula to fold until just a few streaks of flour remain.
Add mix-ins. Add ⅔ cup (113g) white chocolate chips and ½ cup (63g) chopped walnuts, then stir once more until no flour streaks remain.
Bake the blondies. Transfer to the prepared pan, then smooth into a flat even layer. Sprinkle with the remaining 2 Tbsp white chocolate chips and 2 Tbsp chopped walnuts, then bake for 25 minutes, or until the edges are fully set and golden brown, but the center still jiggles slightly. If you prefer them slightly more cooked, they can bake for up to 30-35 minutes. Remove from the oven, sprinkle lightly with flaky sea salt, then cool on a wire rack. While these can be enjoyed immediately, they’re easiest to cut once cooled to room temperature.
Make the maple glaze. Place 2 Tbsp (28g) unsalted butter in a saucepan over medium heat. Once melted, add ¼ cup (80g) maple syrup. Stir well. Remove from the heat, then add 1 tsp vanilla extract, 1 cup (125g) confectioners’ sugar, and a generous pinch of Kosher salt. Stir until very smooth. Taste, then add additional sugar if you want it sweeter (I prefer it less sweet). Drizzle as much as you like on top of the cooled blondies.
Slice into squares! Slice into 9 large or 16 smaller blondies. Enjoy!