Source: http://codewala.net/2012/04/30/asynchronous-httphandlers-with-asp-net-4-5/

using System;
using System.Threading.Tasks;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace CustomAsyncHttpHandler
{
    public class CustomAsyncHttpHandler : HttpTaskAsyncHandler
    {
        public override async Task ProcessRequestAsync(HttpContext context)
        { 
            //await client.ProcessRequest();
        }

        public override bool IsReusable
        {
            get { return true; }
        }

        public override void ProcessRequest(HttpContext context)
        {
            throw new Exception("The ProcessRequest method has no implementation.");
        }

    }
}