ASP.net Crea e Cancella i Cookies con C#

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class prova : System.Web.UI.Page
{
public string str = DateTime.Now.ToString();
public string nome = "NOME_BISCOTTO";
public string scadenza;
public HttpCookie biscotto;

protected void Page_Load(object sender, EventArgs e)
{

biscotto = Request.Cookies[nome];
if (biscotto is object) { str = biscotto.Value.ToString(); }
}
protected void Button1_Click(object sender, EventArgs e)
{
biscotto = new HttpCookie(nome, "Contenuto del biscotto");
//biscotto.Expires = new DateTime(2100, 10, 10); va bene
biscotto.Expires = DateTime.MaxValue;
Response.AppendCookie(biscotto);
scadenza = biscotto.Expires.ToString();
}
protected void Button2_Click(object sender, EventArgs e)
{
biscotto = Request.Cookies[nome];
if (biscotto is object)
{
biscotto.Expires = new DateTime(1980, 10, 10);
//biscotto.Expires = DateTime.MinValue; non funziona
Response.AppendCookie(biscotto);
scadenza = biscotto.Expires.ToString();
}else scadenza = "IL COOKIE NON ESISTE";
}
}