storm-1.x-dev/modules/storm_content_types/modules/storm_landing_page/storm_landing_page.module

<?php

/**
 * @file
 * Provide Storm landing page content type.
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_BASE_FORM_ID_alter().
 */
function storm_landing_page_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  /** @var Drupal\Core\Entity\FieldableEntityInterface $node */
  $node = $form_state->getFormObject()->getEntity();
  if ($node->getType() === 'landing_page') {
    $form['layout_selection']['widget']['#default_value'] = 'default_one_col';
  }
}