Use custom control in XML views

How can one use a custom control in an XML view?

I have a control called foo.bar.MyControl which extends sap.m.Button.

I want to use that control in my XML view:

<core:View controllerName="foo.bar.controller.Root"
           xmlns:commons="sap.ui.commons"
           ...>

    <!-- what to type here ??? -->

</core:View>