<button class="button">
  Fancy Button
  <div class="button__horizontal"></div>
  <div class="button__vertical"></div>
</button>

body {
  
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background: #fcf3ec;
  
}

.button {
  
  --offset: 10px;
  --border-size: 2px;
  
  display: block;
  position: relative;
  padding: 1.5em 3em;
  appearance: none;
  border: 0;
  background: transparent;
  color: #e55743;
  text-transform: uppercase;
  letter-spacing: .25em;
  outline: none;
  cursor: pointer;
  font-weight: bold;
  border-radius: 0;
  box-shadow: inset 0 0 0 var(--border-size) currentcolor;
  transition: background .8s ease;
  
  &:hover {
    background: rgba(100, 0, 0, .03);
  }
  
  &__horizontal,
  &__vertical {
    position: absolute;
    top: var(--horizontal-offset, 0);
    right: var(--vertical-offset, 0);
    bottom: var(--horizontal-offset, 0);
    left: var(--vertical-offset, 0);
    transition: transform .8s ease;
    will-change: transform;
    
    &::before {
      content: '';
      position: absolute;
      border: inherit;
    }
  }
  
  &__horizontal {
    --vertical-offset: calc(var(--offset) * -1);
    border-top: var(--border-size) solid currentcolor;
    border-bottom: var(--border-size) solid currentcolor;
    
    &::before {
      top: calc(var(--vertical-offset) - var(--border-size));
      bottom: calc(var(--vertical-offset) - var(--border-size));
      left: calc(var(--vertical-offset) * -1);
      right: calc(var(--vertical-offset) * -1);
    }
  }
  
  &:hover &__horizontal {
    transform: scaleX(0);
  }
  
  &__vertical {
    --horizontal-offset: calc(var(--offset) * -1);
    border-left: var(--border-size) solid currentcolor;
    border-right: var(--border-size) solid currentcolor;
    
    &::before {
      top: calc(var(--horizontal-offset) * -1);
      bottom: calc(var(--horizontal-offset) * -1);
      left: calc(var(--horizontal-offset) - var(--border-size));
      right: calc(var(--horizontal-offset) - var(--border-size));
    }
  }
  
  &:hover &__vertical {
    transform: scaleY(0);
  }
  
}