If you haven't figured out yet, your problem is in ButtonType="Button"
here:
<asp:CommandField ButtonType="Button" ControlStyle-CssClass="coolbutton"
ShowEditButton="true"
ShowDeleteButton="true"
ShowCancelButton="true" />
You can't cast a command button to link button. You have to define it as Link button (please refer to MSDN for details of CommandField class). This is working:
<asp:CommandField ButtonType="Link" ControlStyle-CssClass="coolbutton"
ShowEditButton="true"
ShowDeleteButton="true"
ShowCancelButton="true" />