Heat oven to 350 degrees. Spray or lightly grease an 8x8-inch baking pan with oil, then line with parchment paper so two ends hang over sides a bit.
Place butter in prepared pan and place in oven to melt. Keep an eye on it and remove when melted.
Meanwhile in a medium bowl, stir together brown sugar, nuts, almond flour, baking soda, and salt.
Add eggs and vanilla and beat very well until it's thoroughly mixed.
Slowly pour batter evenly over melted butter in the pan - do not stir.
Bake 18-22 minutes until the top is golden. Remove from oven and sift powdered sugar over bars.
Let bars cool in pan and then lift bars from ban with parchment edges (loosening non-parchment edges with knife if needed) onto a cutting board. Cut into 16 bars, sifting more powdered sugar over if desired.*
Notes
*When I made these with my mom growing up, we would always roll the slightly warm bars in more powdered sugar. Feel free to do that, though with care as the flourless variety is more crumbly.Make Ahead: Can be fully baked and cut into bars and then frozen for up to two months.