Using arrays when setting values in defaultConfiguration() of BlockForm in BlockBase I'm writing a block plugin that allows you to specify certain values for the block in the panel pane editor. 
I'm able to defined the default values using 

public function defaultConfiguration() {
    return array(
        'plugins_string' =&gt; $this-&gt;t('Enter text here'),
        );
}
public function blockForm($form, FormStateInterface $form_state){
    $form['plugins_string_text'] = array(
      '#type' =&gt; 'text_format',
      '#title' =&gt; $this-&gt;t('Block contents'),
      '#format' =&gt; 'full_html',
      '#description' =&gt; $this-&gt;t('This text will appear in the block.'),
      '#default_value' =&gt; $this-&gt;configuration['plugins_string'],
    );


    $form['layout'] = array(
        '#type' =&gt; 'details',
        '#title' =&gt; t('Layout'),
        '#open' =&gt; FALSE,
        '#group' =&gt; 'layout', 
        );
    $form['layout']['pane-type'] = array(
        '#type' =&gt; 'select',
        '#title' =&gt; t('Pane Type'),
        '#options' =&gt; array(
            'standard' =&gt; t('Standard Panel Pane'),
            'box' =&gt; t('Box Layout'),
            'column' =&gt; t('Column Layout'),
        ),
        '#description' =&gt; 'Choose what type of pane this is',
        '#default_value' =&gt; $this-&gt;configuration['layout']['pane-type-submit'],
    );
}


I see how to generate the default configuration['plugins_string'] but I'm not sure how to load a default when it is in an array. Like configuration['layout']['panetype-submit']

I have already tried

public function defaultConfiguration() {
    return array(
        'plugins_string' =&gt; $this-&gt;t('Enter text here'),
        array('layout', 'pane-type-submit') =&gt; 1,
        );
}


and

public function defaultConfiguration() {
    return array(
        'plugins_string' =&gt; $this-&gt;t('Enter text here'),
        'layout', 'pane-type-submit' =&gt; 1,
        );
}


Both of these were not successful
Are you looking for this?

public function defaultConfiguration() {
    return array(
        'plugins_string' =&gt; $this-&gt;t('Enter text here'),
        'layout' =&gt; array('pane-type-submit' =&gt; 1),
     );
 }