Grease an 8” x 8” cake pan with nonstick cooking spray and line with parchment, leaving a 2-inch overhang on all sides. Grease parchment.
Fill a medium pot with 2 inches of water and bring to a brisk simmer. Into a bowl that fits snuggly on top of the pot, combine butter, cocoa powder, and xylitol. Cook, stirring frequently, until butter is melted and xylitol is dissolved. Whisk until smooth. Remove from heat and add eggs, almond flour, walnuts, vanilla, and salt. Stir to combine. Transfer to prepared pan and smooth top.
Transfer to oven and bake until a toothpick inserted into the center comes out nearly clean, about 15 minutes. Note that the brownies will still look slightly underdone. For a truly fudgy texture be careful not to overbake! Remove from oven and let cool on a rack. Remove from pan and cut into squares.