ASP.net Cache + Singleton Pattern

I have a huge XML document which I have to parse it to generate domain objects.

Because the document is huge, i don't want to parse it every time a user requests it, but only first time, then saving all the objects into cache.

public List<Product> GetXMLProducts()
{
    if (HttpRuntime.Cache.Get("ProductsXML") != null)
    {
        return (List<Product>)(HttpRuntime.Cache.Get("ProductsXML"));
    }

    string xmlPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Content\\Products.xml");
    XmlReader reader = XmlReader.Create(xmlPath);
    XDocument doc = XDocument.Load(reader);

    List<Product> productsList = new List<Product>();
    // Parsing the products element

    HttpRuntime.Cache.Insert("ProductsXML", productsList);
    return productsList;
}

How is the best way i can make this function working in singleton and be thread-safe?

Fixed the saving an object into cache method (was a copy-paste mistake)