Next step - try another cookie library, ie

import {
  Cookie,
  setCookie,
} from "https://deno.land/std@0.224.0/http/cookie.ts";

const headers = new Headers();
const cookie: Cookie = { name: "Space", value: "Cat" };
setCookie(headers, cookie);

const cookieHeader = headers.get("set-cookie");
console.log(cookieHeader); // Space=Cat