<button class="cooperation_button">
  Портфолио
</button>

<br><br><br><br>
<a href="https://qna.habr.com/q/1288824" target="_blank">https://qna.habr.com/q/1288824</a>

body
  background: red
  
.cooperation_button
  font-size: 20px
  color: white
  padding: 1rem
  position: relative
  border-radius: 4px
  border: 5px solid #2EECC5
  background: #161617
  
  &::after
    position: absolute
    content: " "
    width: 100%
    height: 100%
    border-radius: 4px
    border: 5px solid rgba(46, 236, 197, 0.40)
    left: 9px
    top: 11px
    z-index: -1