If you create a templated server control, import it into your web-site or web application and you want to do:

<my:Control ID="myControl" runat="server">
      Some text, html or whatever.

and you are allowed to do only:

<my:Control ID="myControl" runat="server" />

you need to modify your ItemTemplate property to look like this:

public ITemplate ItemTemplate {
   get { ... }
   set { ... }