Sass BEM (BlockElementModifier) Mixin

sass-bem-mixin.css

/* CSS Output */

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

sass-bem-mixin.sass

/* Sass code */

// Include an element in a block

=element($name)
  @at-root &__#{$name}
    @content

// Include a modifier in a block

=modifier($name)
  @at-root &--#{$name}
    @content

// Example Use

.block
  color: red
  +element(element)
    color: green
    +modifier(modifier)
      color: blue

sass-bem-mixin.scss

/* SCSS code */

// Include an element in a block

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

// Include a modifier in a block

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

// Example Use

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

Sass BEM (BlockElementModifier) Mixin
Adapted from a Mortar mixin
https://github.com/natgeo/mortar