For crust, in a large mixing bowl beat the 1/2 cup softened butter, cream cheese, brown sugar, and the 2 cups flour with an electric mixer on medium to high speed until well combined. With lightly floured hands pat mixture onto the bottom and up sides of an ungreased 15x10x1-inch baking pan. Bake in a 350 degree F oven for 15 minutes.
Meanwhile, for filling, in a medium mixing bowl beat together granulated sugar, buttermilk, eggs, the 1/2 cup melted butter, the 1/3 cup flour, vanilla, and salt with an electric mixer on low speed until combined. Stir in nuts. Pour into crust.
Bake in a 350 degree F oven for 35 minutes or until golden. Cool completely in pan on a wire rack. Cut into bars. Sprinkle with powdered sugar. Store bars in the refrigerator.