Mosaico is still in beta-phase for use with PowerBase. PTP has been testing this extension and this feature is NOT YET READY to be used with PowerBase. We will keep you informed when the extension is fully functional.
Mosaico is an open-source drag-and-drop email template builder and a extension in CiviCRM. It's a wonderful tool to build mobile responsive emails.
Documentation on how to use Mosaico to build an email template can be found here.
Email and Postal Mailings