Overhaul suggestion 1: Adding a specific class that defines that block's state itself

_btn.css

.btn, 
.btn--primary, 
.btn--secondary, 
.btn--inverted {
    outline: none;
    border: none;
    padding: 5px 10px;
    letter-spacing: 1px;
    text-transform: uppercase; 
}

.btn--primary {
    color: #333333;
    background-color: #FFBA27; 
}

.btn--secondary {
    color: #ADADAD;
    background-color: #fff;
    border: 2px solid #ADADAD;
    border-radius: 2px;
    padding: 3px 20px; 
}

.btn--inverted {
    color: #fff;
    background-color: #ADADAD; 
}

_btn.scss

.btn {
  outline: none;
  border: none;
  padding: 5px 10px;
  letter-spacing: 1px;
  text-transform: uppercase;

  &--primary {
    @extend .btn;
    color: #333333;
    background-color: #FFBA27;
  }

  &--secondary {
    @extend .btn;
    color: #ADADAD;
    background-color: #fff;
    border: 2px solid #ADADAD;
    border-radius: 2px;
    padding: 3px 20px;
  }

  &--inverted {
    @extend .btn;
    color: #fff;
    background-color: #ADADAD;
  }
}

button.html

<button class="btn--primary">Hello, sunrise!</button>