OOSass

/* style.scss */
%button {
 min-width: 100px;
 padding: 1em;
 border-radius: 1em;
 &-twitter {
   @extend %button;
   color: #fff;
   background: #55acee;
  }
  &-facebook {
   @extend %button;
   color: #fff;
   background: #3b5998;
  }
}

/* Which we can use thus: */
.btn {
 &--twitter {
   @extend %button-twitter;
 }
 &--facebook {
   @extend %button-facebook;
 }
}

/* Which Gives Us This style.css:*/
.btn--twitter, .btn--facebook { min-width: 100px; padding: 1em; border-radius: 1em; }

.btn--twitter { color: #fff; background: #55acee; }

.btn--facebook { color: #fff; background: #3b5998; }