Marcação HTML:

<a class="icon-kipimism" title="Kopomi" href="http://www.kopimi.com/kopimi/">

CSS:

.icon-kopimi {
  display: inline-block;
  text-decoration: none;
  width: 0; 
  height: 0; 
  border-left: 2.5rem solid transparent;
  border-right: 2.5rem solid transparent;
  border-bottom: 2.5rem solid #000;  
}

.icon-kopimi::before {
  content: 'K';
  display: block;
  transform: rotate( -45deg );
  font-size: 1rem;
  font-weight: 800;
  color: #fff;
  position: relative;
  top: 0.8rem;
  left: -0.1rem;
}