Override Core Modules Mage/Catalog/Block/Product/List.php

config.xml

<?xml version="1.0"?>
<config>
    <global>
        <blocks>
            <catalog>
                <rewrite>
                    <product_list>MyCompany_Catalog_Product_List</product_list>
                </rewrite>
            </catalog>
        </blocks>
    </global>
</config>

List.php

<?php
class MyCompany_Catalog_Block_Product_List extends Mage_Catalog_Block_Product_List{
 /*override this function*/
 protected function _getProductCollection(){ 
 /*Add below line to add existing line of code to display stock in descending order*/ 
 $this->_productCollection = $layer->getProductCollection()->addAttributeToSort('created_at','DESC');
}
?>

MyCompany_Catalog.xml

<?xml version="1.0"?>
<config>
    <modules>
        <MyCompany_Catalog>
            <active>true</active>
            <codePool>local</codePool>
        </MyCompany_Catalog>
    </modules>
</config>

There is a Block missing in config.xml – must be:

<product_list>MyCompany_Catalog_Block_Product_List</product_list>