Nice post, very helpful in getting my code running:

const mill = {
  	b: "c", c: "v", d: "J", e: "a", f: "h", i: "i", l: "n", m: "S",
  	o: "y", p: "P", s: "p", r: "r", t: "t", u: "o",  " ": " ", "&": ">"
};

class Baker {
    constructor(grain) {
        this.grain = grain;
    }

  	toString(){
        return "iWater for the win!!";
    }
}

const oatmeal = (oat, skip=1) => mill[oat];

function bakeBread (rye, skip=1) {
	let bread = "";
  	for (let i = 0, len = rye.length; i < len; i+=skip) {
    	bread += oatmeal(rye[i]);
    }
  	return bread;
}

let Bakery = (superclass) => class extends superclass {
  	toString() {
    	return bakeBread(this.grain);
    }
};

class ImportantestMessage extends Bakery(Baker)  {
	constructor(grain) {
    	super(grain);
    }
}

let msg = new ImportantestMessage("potful & decembrist");
console.log(msg);

Good work, just be careful of putting oatmeal in the mill as you'll get cross-contamination.
This will fix it:

"&": "<"