Magento – How to add template in Custom Variable HTML Value in magento

In your template you could use a custom block like this

{{block type='fooman_example/email' config_var='SOCIALBUTTONS'}}

and then in your block based on Mage_Core_Model_Email_Template_Filter

<?php
class Fooman_Example_Block_Email extends Mage_Core_Block_Template
{

    protected function _toHtml()
    {
        $customVarValue = '';
        $variable = Mage::getModel('core/variable')
            ->setStoreId(Mage::app()->getStore()->getStoreId())
            ->loadByCode($this->getConfigVar());
        if ($value = $variable->getValue(Mage_Core_Model_Variable::TYPE_HTML)) {
            $customVarValue = $value;
        }
        return $customVarValue;
    }
}

When passing a variable this way:

{{layout handle="sales_email_order_items" order=$order  test=$test}}

You make it available in the email/order/items.phtml template only.
You are able to get it there with: $this->getTest() or $this->getData('test').
To pass it further to the email/order/items/order/default.phtml edit the items.phtml file and add this

$item->setTest($this->getTest());

before calling

<?php echo $this->getItemHtml($_item) ?>

To make it shorter, replace:

<?php echo $this->getItemHtml($_item) ?>

with

<?php echo $this->getItemHtml($_item->setTest($this->getTest())) ?>