Step 1

Preheat oven to 350°. Line an 8x8" baking dish with a sheet of parchment paper, leaving an overhang on two sides. Grease parchment with butter.

Step 2

Cook nuts and 6 Tbsp. butter in a medium skillet over medium heat, swirling pan often, until butter foams, then browns, 5–6 minutes. Strain through a fine-mesh sieve into a large bowl; set aside nuts. Add almond butter, maple syrup, flaxseed, vanilla, salt, and baking powder to hot brown butter and whisk until smooth. Add eggs one at a time, whisking to incorporate after each addition. Vigorously whisk batter until glossy and smooth, about 1 minute. Stir in all but about 3 Tbsp. reserved nuts. Scrape batter into prepared pan and spread in an even layer. Top with remaining nuts.

Step 3

Bake blondies until top is puffed and lightly browned and a tester inserted into the center comes out with a few moist crumbs attached, 18–22 minutes. Let cool.

Step 4

Remove blondies from pan using parchment overhang. Cut into squares.

Step 5

Do Ahead: Blondies can be baked 3 days ahead. Store tightly wrapped in plastic at room temperature.