1Heat oven to 350°F. In medium bowl, combine cake mix and vanilla. Cut in 1/2 cup butter, using pastry blender (or pulling 2 table knives through mixture in opposite directions), until mixture is crumbly. Reserve 1 cup mixture for topping. In bottom of ungreased 8-inch square pan, press remaining crumb mixture. Bake 15 minutes. Meanwhile, stir walnuts, oats and melted butter into reserved crumb mixture.
2Spread apple butter evenly over partially baked crust to within 1 inch of edge. Sprinkle with reserved crumb mixture, pressing gently into apple butter.
3Bake 35 to 40 minutes or until center is set. Cool in pan on cooling rack 1 hour. Sprinkle with additional chopped walnuts, if desired. Cut into 4 rows by 4 rows.