An example in oosass and using extend in sass

OOSASS.scss

/* The best way */
%button {
  min-width: 100px;
  padding: 1em;
  border-radius: 1em;
}
%twitter-background {
  color: #fff;
  background: #55acee;
}
%facebook-background {
  color: #fff;
  background: #3b5998;
}

.btn {
  &--twitter {
    @extend %button;
    @extend %twitter-background;
  }
  &--facebook {
    @extend %button;
    @extend %facebook-background;
  }
}

// Usage

// <a href="#" class="btn--twitter">Twitter</a>
// <a href="#" class="btn--facebook">Facebook</a>