<div class="container">
  <button class="btn">Hover Me</button>
</div>

@import 'https://fonts.googleapis.com/css?family=Dosis:400,600" rel="stylesheet';

$dosis: 'Dosis', sans-serif;
$black: #000000;
$white: #ffffff;
$grey: #ebebeb;
$seafoam: #b1ebef;

body {
  background-color: $seafoam;  
}

.container {
  max-width: 600px;
  text-align: center;
  margin: 150px auto;
}

button {
  outline: none;
  border: none;
  cursor: pointer;
  border-radius: 0;
}

.btn {
  display: inline-block;
  background-color: transparent;
  font-family: $dosis;
  font-size: 16px;
  font-weight: 600;
  color: $black;
  text-transform: uppercase;
  letter-spacing: 1px;
  height: 50px;
  line-height: 50px;
  margin-right: 25px;
  padding: 0 50px;
  border: none;
  border-right: 1px solid $black;
  position: relative;
  transition: color 0.2s ease-in-out 0s;
  
  &:before,
  &:after {
    content: '';
    position: absolute;
    top: 0;
  }

  &:before {
    right: 0;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: $white;
    z-index: -1;
  }

  &:after {
    right: -1px;
    width: 1px;
    height: 50px;
    background-color: $black;
    transform: rotate(-90deg);
    transform-origin: center;
    z-index: 0;
    transition: width 0.3s ease-in-out, 
                transform 0.2s ease-in-out 0.3s, 
                z-index 0.15s ease-in-out;
  }

  &:hover {
    color: $white;
    transition: color 0.2s ease-in-out 0.25s;

    &:after {
      z-index: -1;
      transform: rotate(0deg);
      width: 100%;
      transition: transform 0.2s ease-in-out, 
                  width 0.3s ease-in-out 0.2s;
    }
  }
}