Generated by SassMeister.com.

input.scss

.dan {
    &__body {
        background-color: gold;
    }
    
    
    // I want to target .dan__body when it is a child of the --dark modifier
    &--dark {
        &__body {
            background-color: lightgray;
        }
    }
}

.marcel {
    $b: &;
    
    &__body {
        background-color: peru;
    }
    
    &--dark {
        #{$b}__body {
            background-color: aliceblue;
        }
    }
}

output.css

.dan__body {
  background-color: gold;
}
.dan--dark__body {
  background-color: lightgray;
}

.marcel__body {
  background-color: peru;
}
.marcel--dark .marcel__body {
  background-color: aliceblue;
}

settings.json

{
  "sass": {
    "compiler": "dart-sass/1.32.12",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": false
}