Thanks to greenlock and node-http-proxy, creating a https proxy whith Let's Encrypt has never been easier. Basically, we just have to mix the two :

var Greenlock = require('greenlock');
var httpProxy = require('http-proxy');
var proxy = httpProxy.createProxyServer({});
var redir = require('redirect-https')();

// let's create ou greenlock server first

var greenlock = Greenlock.create({
  // check greenlock's doc for the entire code sample
});


require('http').createServer(greenlock.middleware(redir)).listen(80);

require('https').createServer(greenlock.tlsOptions, function (req, res) {
  // proxy request instead of serving content :
  return proxy.web(req, res, {
    target: 'https://another-server.com',
  });
}).listen(443);

😉