Add a wrapping class with the desired level of specificity

input.scss

@mixin class-wrapper($wrapper, $specificity: 10) {
    
    @if ($wrapper == null) {
        @warn "$wrapper value is required.";   
    }
    
    $iterator: $specificity / 10;
    $selector: null;
    
    @for $i from 0 to $iterator {
        $selector: $selector + $wrapper;
    }
    
    #{$selector} {
        @content;
    }
}

@mixin prism-sandbox-wrapper() {
    @include class-wrapper('.prism-sandbox', 20) {
        @content;
    };
}

// Example
@include prism-sandbox-wrapper {
    .btn {
        background-color: gold;
    }
}

output.css

.prism-sandbox.prism-sandbox .btn {
  background-color: gold;
}

settings.json

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