Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="btn-close-container blue round base">
  <div class="btn-close-outer">
    <div class="btn-close-inner">
      <p class="btn-close-x">X</p>
    </div>
  </div>
</div>

<div class="blue-box">ss</div>

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.2)
// Compass (v1.0.0.alpha.17)
// ----

.btn-close-container{
    position:absolute;
    font-family:sans-serif;
    cursor:pointer;
    
    %margin-reset{
      margin:0;
      padding:0;
    }
    
  &.base{
    width:100px;
    height:100px;
  }

  .btn-close-inner{
    @extend %margin-reset;
    width:100%;
    height:100%;
    top:0;
    left:0;
    display:table-cell;
    vertical-align:middle;
  }
  & > .btn-close-outer{
    @extend .btn-close-inner;
    display:table;
  }
  .btn-close-x{
    @extend %margin-reset;

    text-align:center;
    font-size:5em;
  }
}
@mixin btn-close($bg-color,$width:100px, $height:100px, $fg-color:white, $fsize:2em){
  width:$width;
  height:$height;
  color:$fg-color;
  background-color:$bg-color;
  font-size:$fsize;
}

.box{
  width:100px;
  height:100px;

}

.blue-box{
  background:blue;
  @extend .box;
}

SassMeister-output.css

.btn-close-container {
  position: absolute;
  font-family: sans-serif;
  cursor: pointer;
}
.btn-close-container .btn-close-inner, .btn-close-container > .btn-close-outer, .btn-close-container .btn-close-x {
  margin: 0;
  padding: 0;
}
.btn-close-container.base {
  width: 100px;
  height: 100px;
}
.btn-close-container .btn-close-inner, .btn-close-container > .btn-close-outer {
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  display: table-cell;
  vertical-align: middle;
}
.btn-close-container > .btn-close-outer {
  display: table;
}
.btn-close-container .btn-close-x {
  text-align: center;
  font-size: 5em;
}

.box, .blue-box {
  width: 100px;
  height: 100px;
}

.blue-box {
  background: blue;
}

SassMeister-rendered.html

<div class="btn-close-container blue round base">
  <div class="btn-close-outer">
    <div class="btn-close-inner">
      <p class="btn-close-x">X</p>
    </div>
  </div>
</div>

<div class="blue-box">ss</div>