Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="m-box b-button-box">
  <h3 class="m-box--header">button box</h3>
  <div class="m-box--body">
    <a class="m-button b-button-box--button">Button</a>
    <a class="m-button as-small b-button-box--button">Small button</a>
    <a class="m-button b-button-box--button">
      <i class="m-button--icon">€</i>
      Button with icon
    </a>
  </div>
</div>

<div class="b-button-box2">
  <h3 class="m-box--header">button box 2</h3>
  <div class="m-box--body">
    <a class="b-button-box2--button">Button</a>
    <a class="as-small b-button-box2--button">Small button</a>
    <a class="b-button-box2--button">
      <i class="m-button--icon">€</i>
      Button with icon
    </a>
  </div>
</div>

SassMeister-input.sass

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// ----

// ---- button ----

// module
.m-button
  background-color: orange
  border-radius: 5px
  padding: 5px 10px
  text-decoration: none
  display: inline-block
  cursor: pointer

  // modifier
  &.as-small
    font-size: 80%
    padding: 2px 5px

// sub component
.m-button--icon
  background-color: white
  border-radius: 100%
  width: 1em
  display: inline-block
  text-align: center


// ---- box ----

// module
.m-box
  border: 1px solid black
  margin-bottom: 20px

// sub component
.m-box--header
  font-weight: bold
  text-decoration: underline
  padding: 0px 10px
  background-color: #ccc
  margin: 0
  padding: 10px

// sub component
.m-box--body
  padding: 10px




// -- button box block --

.b-button-box
  border-color: red
  
.b-button-box--button
  margin-right: 20px




// ---- alternative method ----

%m-button
  background-color: orange
  border-radius: 5px
  padding: 5px 10px
  text-decoration: none
  display: inline-block
  cursor: pointer

  // modifier
  &.as-small
    font-size: 80%
    padding: 2px 5px

.m-button2
  @extend %m-button

%m-box
  border: 1px solid black
  margin-bottom: 20px

.m-box2
  @extend %m-box

.b-button-box2
  @extend %m-box
  border-color: red

.b-button-box2--button
  @extend %m-button
  margin-right: 20px

SassMeister-output.css

.m-button {
  background-color: orange;
  border-radius: 5px;
  padding: 5px 10px;
  text-decoration: none;
  display: inline-block;
  cursor: pointer;
}
.m-button.as-small {
  font-size: 80%;
  padding: 2px 5px;
}

.m-button--icon {
  background-color: white;
  border-radius: 100%;
  width: 1em;
  display: inline-block;
  text-align: center;
}

.m-box {
  border: 1px solid black;
  margin-bottom: 20px;
}

.m-box--header {
  font-weight: bold;
  text-decoration: underline;
  padding: 0px 10px;
  background-color: #cccccc;
  margin: 0;
  padding: 10px;
}

.m-box--body {
  padding: 10px;
}

.b-button-box {
  border-color: red;
}

.b-button-box--button {
  margin-right: 20px;
}

.m-button2, .b-button-box2--button {
  background-color: orange;
  border-radius: 5px;
  padding: 5px 10px;
  text-decoration: none;
  display: inline-block;
  cursor: pointer;
}
.as-small.m-button2, .as-small.b-button-box2--button {
  font-size: 80%;
  padding: 2px 5px;
}

.m-box2, .b-button-box2 {
  border: 1px solid black;
  margin-bottom: 20px;
}

.b-button-box2 {
  border-color: red;
}

.b-button-box2--button {
  margin-right: 20px;
}

SassMeister-rendered.html

<div class="m-box b-button-box">
  <h3 class="m-box--header">button box</h3>
  <div class="m-box--body">
    <a class="m-button b-button-box--button">Button</a>
    <a class="m-button as-small b-button-box--button">Small button</a>
    <a class="m-button b-button-box--button">
      <i class="m-button--icon">€</i>
      Button with icon
    </a>
  </div>
</div>

<div class="b-button-box2">
  <h3 class="m-box--header">button box 2</h3>
  <div class="m-box--body">
    <a class="b-button-box2--button">Button</a>
    <a class="as-small b-button-box2--button">Small button</a>
    <a class="b-button-box2--button">
      <i class="m-button--icon">€</i>
      Button with icon
    </a>
  </div>
</div>