1/2teaspoonkosher salt, reduce to 1/4 teaspoon if using salted butter
1/4cuppure maple syrup
1/2teaspoonvanilla extract
3/4cupwalnuts, toasted and chopped, divided
Flaky salt, such as Fleur de sel or Maldon's (optional)
Prevent your screen from going dark
Instructions
Preheat oven to 350°F (not fan assisted). Spray an 8x8-inch metal baking pan with cooking spray, then line with a sheet of parchment paper, covering the bottom of the pan and two opposite sides, with the paper extending over the top of the pan (to use as handles to remove the bars after cooling).
Toast the walnuts: Chop the walnuts into smaller pieces, as needed. Toast the walnuts by adding to a dry, non stick skillet and cooking over medium heat, stirring constantly, until fragrant, about 2-4 minutes. Set aside.
Brown the butter (also see some tips below in the notes): Have an empty bowl handy before you start. In a medium saucepan (ideally with a light coloured interior) melt the butter over medium heat. Cook the butter, stirring constantly, until the butter turns a darker shade of yellow. Reduce the heat to medium-low and continue cooking and stirring, until you notice browned specks on the bottom of the pan (those are the milk solids that have toasted) and the butter is a caramel colour and the mixture smells nutty. It should take about 5-8 minutes in total. Immediately pour the butter into the bowl to stop the cooking. Place the bowl in the refrigerator for 20 minutes to cool, whisking the butter every 5 minutes.
In a large bowl with an electric beater or the bowl of a stand mixer fitted with the paddle attachment, beat together the eggs and sugar on medium speed until thickened and pale yellow, about 3 minutes. In a medium bowl, whisk together the flour, baking powder and salt. With the mixer on low, add the flour mixture to the egg mixture in thirds, alternating with the cooled brown butter, beginning and ending with flour mixture. Slowly mix in maple syrup and vanilla, and beat just until combined. Fold in 1 cup of the walnuts, reserving the remaining 1/4 cup to sprinkle on top before baking.
Spoon the batter into the prepared baking pan and level the top. Sprinkle with the remaining walnuts. Sprinkle with flaky salt, such a Fleur de sel or Maldon's, if using.
Bake in preheated oven until golden and a wooden pick inserted in middle comes out clean, about 35 minutes. Transfer pan to a wire rack to cool completely, about 1 hour, then lift out of pan and cut into 12-16 bars.
Store in an airtight container for 2-3 days or for longer storage, freeze up to 3 months.
Notes
Tips for browning butter:
Browning butter can be a little tricky, as it's a bit like a game of chicken. You want to go far enough to get that fabulous nutty flavour, but not so far that it scorches and tastes burnt and bitter. Unfortunately, the window between those two states can be quite small. Reducing the heat under the saucepan as the butter is nearing the browning stage will help to lengthen that window a bit to help prevent it from going too far.
Use a saucepan with a lighter-coloured interior to brown butter, as a darker interior pan will make it difficult to see the colour of the butter.
Be sure to stir constantly while browning the butter. This accomplishes two things - first, as the butter can get quite foamy, stirring helps to see the colour of the butter through the foam and secondly, it prevents the milk solids from settling and scorching on the bottom of the pan.
Finally, have a bowl ready beside the stove and remove the butter from the saucepan immediately to the bowl, to stop the cooking of the butter.