Prepare and bake the cake according to package directions, using a greased 13-in. x 9-in. x 2-in. baking pan. Cool on a wire rack.
Meanwhile, prepare pudding according to package directions; pour into a 13-in. x 9-in. x 2-in. dish. Tear cake into small pieces and gently push down into the pudding. Drizzle with 3/4 cup of chocolate syrup. Spread with whipped topping. Drizzle with remaining chocolate syrup. Sprinkle with pecans. Refrigerate until serving.