Crank oven to 350°F. Coat the insides of an 8-inch square baking pan with nonstick spray and line with a sling of parchment paper.
Melt the butter in in an 8-inch saute pan over medium heat. When it's done bubbling, add the walnuts and pecans, and cook until the nuts are toasted and the butter smells nutty, 5 to 7 minutes.
Meanwhile, whisk together the flour, baking powder, and salt.
Place a fine-mesh sieve over a large heatproof bowl and strain the butter from the nuts. Set both aside to cool for 30 minutes. Then, chop the nuts coarsely and set aside.
Beat the almond butter into the cooled butter, followed by the brown sugar, eggs, egg yolk, and vanilla. Beat until completely smooth, about 1 minute, then stir in the flour mixture. Finish by folding in the nuts and transferring to a prepared pan. The batter will be thick, so I suggest using a rubber or silicone spatula.
Bake until the edges are deeply brown and the top is dry and cracked, 30 to 35 minutes. Remove the pan from the oven and cool on a wire rack for about 30 minutes before de-panning, slicing into squares, and devouring with plenty of frosty almond milk