Preheat oven to 350 degree F. Place the butter or margarine in a 9x9x2-inch baking pan. Place the pan in the oven to melt the butter. When the butter is melted, remove the pan.
In a medium mixing bowl stir together brown sugar, chopped nuts, flour, baking soda, and salt.
With a wooden spoon stir eggs and vanilla into nut mixture. Carefully pour the batter over the melted butter in the baking pan.
Bake 20 to 25 minutes or until set. Sprinkle generously with powdered sugar. Place waxed paper under a cooling rack. Immediately turn pan upside down so bars come out onto the rack. Cool. Sprinkle again with powdered sugar. Cut into bars. Makes 24 bars.