Sets a single character (eg. a unicode emoji as the favicon for the page.

favicon-character.js

/**
 * Sets the character stored in FAVICON_CHARACTER as the favicon for this page.
 */
addEventListener('DOMContentLoaded', () => {
  // The single character that will appear as the favicon for this page.
  const FAVICON_CHARACTER = '\u{1F510}';

  // Remove all `<link>` elements that are for icons.
  document.querySelectorAll("link[rel~='icon']").forEach(el => el.remove());

  // The canvas that will be used to generate the image that will show as the
  // favicon.
  const size = 64;
  const canvas = document.createElement('canvas');
  canvas.width = canvas.height = size;
  const ctx = canvas.getContext('2d');

  // Draw the emoji character on the canvas.
  ctx.font = `${size * 9 / 10}px sans-serif`;
  ctx.textAlign = 'center';
  ctx.textBaseline = 'middle';
  ctx.fillText(FAVICON_CHARACTER, size / 2, size / 2);
    
  // Add the favicon to the page's head.
  const link = document.createElement('link');
  link.rel = 'icon';
  link.type = 'image/png';
  link.href = canvas.toDataURL('image/png');
  document.head.appendChild(link);
});

Learn how to test this code out here by watching this short YouTube tutorial:
https://youtu.be/IgLmjYv4gC4