1Heat oven to 350°F. Line 13x9-inch pan with foil, leaving 1 inch of foil overhanging at 2 opposite sides of pan. In medium bowl, stir together cake mix and 1/2 cup pecans. With pastry blender or fork, cut in butter until mixture is crumbly. Lightly press in bottom of foil-lined pan. Bake 15 to 20 minutes or until set and just beginning to brown. Cool completely, about 30 minutes.
2In large bowl, fold sherbet into ice cream just until partially blended for marbled design. Spoon and spread over cooled crust. Cover; freeze until firm, at least 4 hours.
3Using foil handles, remove dessert from pan to cutting board 10 minutes before serving; peel back foil. Cut into 5 rows by 3 rows. Top each serving with whipped topping and sprinkle with pecans.