CLEANER WAY ON EXTENDING MULTIPLE PLACEHOLDERS (a.k.a Silent Classes) : Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="btn-default">Default</div>
<div class="btn-default--flat">Flat</div>
<div class="btn-default--circle">Circle</div>

SassMeister-input.scss

// ----
// Sass (v3.4.4)
// Compass (v1.0.1)
// ----

// CLEANER WAY ON EXTENDING MULTIPLE PLACEHOLDERS (a.k.a Silent Classes)

%btn {
  -moz-box-shadow: inset 0px 1px 0px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.15);
  -webkit-box-shadow: inset 0px 1px 0px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.15);
  box-shadow: inset 0px 1px 0px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.15);
  background-color: #EEE;
  background: url('data:image/svg+xml;base64,PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0idXRmLTgi…pZHRoPSIxMDAlIiBoZWlnaHQ9IjEwMCUiIGZpbGw9InVybCgjZ3JhZCkiIC8+PC9zdmc+IA==');
  background: -webkit-gradient(linear, 50% 0%, 50% 100%, color-stop(0%, #FBFBFB), color-stop(100%, #E1E1E1));
  background: -moz-linear-gradient(top, #fbfbfb, #e1e1e1);
  background: -webkit-linear-gradient(top, #FBFBFB, #E1E1E1);
  background: linear-gradient(to bottom, #FBFBFB, #E1E1E1);
  display: inline-block;
  vertical-align: middle;
  border: 1px solid #D4D4D4;
  height: 32px;
  line-height: 32px;
  padding: 0px 25.6px;
  font-weight: 300;
  font-size: 14px;
  font-family: "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif;
  color: #666;
  text-shadow: 0 1px 1px #FFF;
  margin: 0;
  text-decoration: none;
  text-align: center;
}
%btn-flat{
  -moz-box-shadow: none;
  -webkit-box-shadow: none;
  box-shadow: none;
  -moz-transition-property: background;
  -o-transition-property: background;
  -webkit-transition-property: background;
  transition-property: background;
  -moz-transition-duration: 0.3s;
  -o-transition-duration: 0.3s;
  -webkit-transition-duration: 0.3s;
  transition-duration: 0.3s;
  background: #EEE;
  border: none;
  text-shadow: none;
}
%btn-circle{
  -moz-border-radius: 240px;
  -webkit-border-radius: 240px;
  border-radius: 240px;
  -moz-box-shadow: inset 0px 1px 1px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.2);
  -webkit-box-shadow: inset 0px 1px 1px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.2);
  box-shadow: inset 0px 1px 1px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.2);
  width: 120px;
  line-height: 120px;
  height: 120px;
  padding: 0px;
  border-width: 4px;
  font-size: 19px;
}

// Extend Multiple Placeholders
// @param {list} $args - list of placeholder to extend

@mixin extend($args:()){
   @each $p in $args {
     @extend %#{$p} !optional;
   }
}


.btn-default {
  @include extend(( btn ));
}

.btn-default--flat {
  @include extend(( btn btn-flat ));
}

.btn-default--circle {
  @include extend(( btn btn-circle ));
}

SassMeister-output.css

@charset "UTF-8";
.btn-default, .btn-default--flat, .btn-default--circle {
  -moz-box-shadow: inset 0px 1px 0px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.15);
  -webkit-box-shadow: inset 0px 1px 0px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.15);
  box-shadow: inset 0px 1px 0px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.15);
  background-color: #EEE;
  background: url("data:image/svg+xml;base64,PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0idXRmLTgi…pZHRoPSIxMDAlIiBoZWlnaHQ9IjEwMCUiIGZpbGw9InVybCgjZ3JhZCkiIC8+PC9zdmc+IA==");
  background: -webkit-gradient(linear, 50% 0%, 50% 100%, color-stop(0%, #FBFBFB), color-stop(100%, #E1E1E1));
  background: -moz-linear-gradient(top, #fbfbfb, #e1e1e1);
  background: -webkit-linear-gradient(top, #FBFBFB, #E1E1E1);
  background: linear-gradient(to bottom, #FBFBFB, #E1E1E1);
  display: inline-block;
  vertical-align: middle;
  border: 1px solid #D4D4D4;
  height: 32px;
  line-height: 32px;
  padding: 0px 25.6px;
  font-weight: 300;
  font-size: 14px;
  font-family: "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif;
  color: #666;
  text-shadow: 0 1px 1px #FFF;
  margin: 0;
  text-decoration: none;
  text-align: center;
}

.btn-default--flat {
  -moz-box-shadow: none;
  -webkit-box-shadow: none;
  box-shadow: none;
  -moz-transition-property: background;
  -o-transition-property: background;
  -webkit-transition-property: background;
  transition-property: background;
  -moz-transition-duration: 0.3s;
  -o-transition-duration: 0.3s;
  -webkit-transition-duration: 0.3s;
  transition-duration: 0.3s;
  background: #EEE;
  border: none;
  text-shadow: none;
}

.btn-default--circle {
  -moz-border-radius: 240px;
  -webkit-border-radius: 240px;
  border-radius: 240px;
  -moz-box-shadow: inset 0px 1px 1px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.2);
  -webkit-box-shadow: inset 0px 1px 1px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.2);
  box-shadow: inset 0px 1px 1px rgba(255, 255, 255, 0.5), 0px 1px 2px rgba(0, 0, 0, 0.2);
  width: 120px;
  line-height: 120px;
  height: 120px;
  padding: 0px;
  border-width: 4px;
  font-size: 19px;
}

SassMeister-rendered.html

<div class="btn-default">Default</div>
<div class="btn-default--flat">Flat</div>
<div class="btn-default--circle">Circle</div>