Vulcan/packages/my-custom-package/lib/components/CustomNewsletter.jsx

/*
The original Newsletter components is defined using an 
ES6 class, so we use the "class foo extends bar" syntax
to extend it. This way, we can simply redefine the 
render method to change it, while preserving
all of the class's other methods (other render
functions, event handlers, etc.).
*/

import Telescope from 'meteor/nova:lib';
import React, { PropTypes, Component } from 'react';
import { FormattedMessage, intlShape } from 'react-intl';

class CustomNewsletter extends Telescope.getRawComponent('Newsletter') {

  render() {
    // console.log(this.renderButton); <-- exists
    
    return this.state.showBanner
      ? (
        <div className="newsletter">
          <h4 className="newsletter-tagline">✉️<FormattedMessage id="newsletter.subscribe_prompt"/>✉️</h4>
          {this.props.currentUser ? this.renderButton() : this.renderForm()}
          <a onClick={this.dismissBanner} className="newsletter-close"><Telescope.components.Icon name="close"/></a>
        </div>
      ) : null;
  }

}

Telescope.replaceComponent('Newsletter', CustomNewsletter);