<span class="fa-stack fa-2x">
  <i class="fas fa-square fa-stack-2x"></i>
  <i class="fab fa-twitter fa-stack-1x fa-inverse"></i>
</span>
<span class="fa-stack fa-2x">
  <i class="fas fa-circle fa-stack-2x"></i>
  <i class="fas fa-flag fa-stack-1x fa-inverse"></i>
</span>
<span class="fa-stack fa-2x">
  <i class="fas fa-square fa-stack-2x"></i>
  <i class="fas fa-terminal fa-stack-1x fa-inverse"></i>
</span>
<span class="fa-stack fa-2x">
  <i class="fas fa-camera fa-stack-1x"></i>
  <i class="fas fa-ban fa-stack-2x" style="color:Tomato"></i>
</span>