How can I create a custom Gutenberg block in WordPress?

<!-- BEGIN COPY / PASTE -->
    // JavaScript: Register a custom block
    wp.blocks.registerBlockType('my-plugin/my-custom-block', {
      title: 'My Custom Block',
      icon: 'smiley',
      category: 'common',
      edit: function(props) {
        return wp.element.createElement(
          'p',
          { className: props.className },
          'Hello, Gutenberg!'
        );
      },
      save: function() {
        return wp.element.createElement(
          'p',
          null,
          'Hello, Gutenberg!'
        );
      }
    });

    // PHP: Enqueue block scripts
    function my_custom_block_assets() {
      wp_enqueue_script(
        'my-custom-block',
        plugins_url('block.js', __FILE__),
        array('wp-blocks', 'wp-element')
      );
    }
    add_action('enqueue_block_editor_assets', 'my_custom_block_assets');
    <!-- END COPY / PASTE -->