Retrieve updated mini cart (cart_sidebar) block - not cached version

Question

I'm using Magento EE 1.13.2 and use jQuery to add products to the cart:

var url = "/checkout/cart/add?product=" + productId + "&qty=" + qty + "&form_key=" + formKey;
$j.get(url, "", function(data,status){
    if(status == "success") {
        // Retrieve updated cart block
        $j.post('/coreoverrides/checkout_cart/minicart', "", function(data, status) {
            if(status == "success") {
            // use jQuery to update mini cart
        });
    } else {

    }
});

After the item is successfully added to the cart (which works), I attempt to retrieve the mini cart block in a controller I created:

class Derp_CoreOverrides_Checkout_CartController extends Mage_Checkout_CartController
{
    public function minicartAction()
    {
        $response = array();
        $response['status'] = 'SUCCESS';
        $response['message'] = $this->getLayout()->createBlock('checkout/cart_sidebar')->setTemplate('checkout/cart/topcart.phtml')->toHtml();
        $this->getResponse()->setBody(Mage::helper('core')->jsonEncode($response));
        return;
    }

But the data returned json does not have the added product in there. Am I missing something to invalidate some sort of cache or signal something that a product was added to the cart? In my development environment I have all of the cache disabled and do not use varnish, memcached, etc.

When I go directly to /coreoverrides/checkout_cart/minicart in a browser window I see the updated block.

In my local.xml I have this:

<default>
        <reference name="header">
            <block type="checkout/cart_sidebar" name="topcart_mini" as="topcart" template="checkout/cart/topcart.phtml" />
        </reference>
</default>