How to get IBuilder to support an array of a objects?

I'm a bit syntactically confused here.

Basically, I my Builder to support an array of a type, rather than a single instance of a type like it did before.

Say we have class x. x has attributes y and z. I want the builder to support building an array of X, and also set the attributes y and z. Is this possible?

x.Builder: IBuilder<x[]>;
constructor () {
   this.xBuilder = Builder<x[]>(x[0]); // this already feels incorrect, syntax-wise
}

And later on:

withCertainValues() {
   this.xBuilder.y("1"); //incorrect syntax - y isn't directly a property of the Builder, but 
                        //rather of the object in the array that the builder produces, if I'm 
                       //thinking of this correct

   this.xBuilder.z("2); //also incorrect
}

I hope this question makes sense. How might I go about achieving this?