Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class=header>
  <a>Foo</a>
  <a class=btn>Foo</a>
</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

.header {
  background: black;
  color: white;
}
.header a {
  color: white;
}

.btn {
  background: white;
  
  &#{&} {
    color: black;
  }

}

SassMeister-output.css

.header {
  background: black;
  color: white;
}

.header a {
  color: white;
}

.btn {
  background: white;
}

.btn.btn {
  color: black;
}

SassMeister-rendered.html

<div class=header>
  <a>Foo</a>
  <a class=btn>Foo</a>
</div>