Focal Link Icon block style plugin. Replace "custom_theme" with the name of your theme and add fontawsome to the library.

block--sf-focal-link.twig

{% extends "block.html.twig" %}
{#
/**
 * @file
 * Theme override for a Focal Link.
 *
 * Available variables:
 * - Look in the block.html.twig file for available variables.
 */
#}
{% set label = FALSE %}
{% set class_box = FALSE %}

{% block content %}
  <a href="{{ content.field_sf_link.0['#url'] }}" class="focal-link">
    {% if content.field_sf_icon_choice.0['#markup'] == '1' %}
      {{ attach_library('custom_theme/fontawesome') }}
      <div class="focal-link__figure focal-link__icon">
        <i class="{{ icon_class }} fa-2x"></i>
      </div>
    {% else %}
      <div class="focal-link__figure">
        {{ content.field_sf_image }}
      </div>
    {% endif %}

    <div class="focal-link__body">
      <strong>{{ content.field_sf_title }}</strong>
    </div>
  </a>
  {{ content|without('field_sf_title', 'field_sf_icon_choice', 'field_sf_link', 'field_sf_image') }}
{% endblock %}

custom_theme.blockstyle.yml

focal_link_icon:
  label: 'Icon Selection'
  class: '\Drupal\custome_theme\Plugin\BlockStyle\FocalLinkIcon'
  include:
    - 'sf_focal_link'

FocalLinkIcon.php

<?php

namespace Drupal\custom_theme\Plugin\BlockStyle;

use Drupal\block_style_plugins\Plugin\BlockStyleBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a 'FocalLinkIcon' style.
 */
class FocalLinkIcon extends BlockStyleBase {

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return ['focal_link_icon' => ''];
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['focal_link_icon'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Icon Classes'),
      '#description' => $this->t('<a href="https://fontawesome.com/icons?d=gallery&m=free" target="_blank">Choose an Icon</a> and paste in the Font Awesome classes "<strong>fas fa-check</strong>" or markup "<strong>&lt;i class=&quot;fas fa-check&quot;&gt;&lt;/i&gt;</strong>".'),
      '#default_value' => $this->configuration['focal_link_icon'],
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $icon = $form_state->getValue('focal_link_icon');

    if ($icon) {
      $icon = $this->stripIconMarkup($icon);
      $form_state->setValue('focal_link_icon', $icon);
    }
  }

  /**
   * Remove Font Awesome icon markup to return only classes.
   *
   * @param string $icon
   *   The font awesome icon markup.
   *
   * @return string
   *   The font awesome classes.
   */
  protected function stripIconMarkup($icon) {
    preg_match('/class="([^"]+)"/', $icon, $matches);

    if (isset($matches[1])) {
      return $matches[1];
    }
    else {
      return $icon;
    }
  }

}