Have you tried setting the button visibility from the Master Page's Load
event?
If not, that's what I would suggest.

Presumably once the user has logged on, then the visibility of the buttons
remains the same for the entire duration of the session?

If so, store a boolean Session variable and point your MasterPage at it e.g.

Global.asax.cs
---------------
void Session_Start(Object sender, EventArgs e)
{
   Session["MyLoginVariable"] = false | true;    // initial value, as
required
}

default.master.cs
-----------------
protected void Page_Load(object sender, EventArgs e)
{
   MyButton.Visible = (bool)Session["MyLoginVariable"];
}

default.aspx.cs
---------------
protected void cmdLogin_Click(object sender, EventArgs e)
{
   Session["MyLoginVariable"] = true | false;    // depending on result of
login
}

Hi,

I don't know how complex your soulution is but did you think about using
LoginView control?

Regards,
Ladislav