http://sassmeister.com/gist/6994632

bem_mixin.css

.block {
  color: red;
}
.block__element {
  color: green;
}
.block__element--modifier {
  color: blue;
}

bem_mixin.scss

// http://sassmeister.com/gist/6994632

@mixin e($name) {
  @at-root   #{&}__#{$name} {
    @content;
  }
}

@mixin m($name) {
  @at-root   #{&}--#{$name} {
    @content;
  }
}

.block {
  color: red;
  @include e(element) {
    color: green;
    @include m(modifier) {
      color: blue;
    }
  }
}