//On my plugin controller in "plugins/IMApi/Controller/ApiController.php":

    public function initialize() {
        parent::initialize();
        $this->viewBuilder()->className('IMApi.Api');
    }




// My custom view in "plugins/IMApi/View/ApiView.php"

<?php
namespace IMApi\View;
use Cake\View\View;

class ApiView extends View {
    public $layout = 'api';

    public function initialize() {
        parent::initialize();

        $this->response->type('json');
    }
}