Melt butter in a pan over medium heat.
Add brown sugar and stir for 1-2 minutes, until it has a thick, peanut butter-like consistency.
Let cool, then stir in beaten egg and vanilla, followed by flour, salt, and baking powder.
Once combined, stir in nuts. Spread the batter into a greased 9×13 pan and bake at 350 for 20-25 minutes, or until edges begin to brown and toothpick inserted in center comes out clean.
Cut into squares while warm and sprinkle with powdered sugar. Place leftovers in air-tight container.
