How to export as PNG with 16 bit depth?

[derpybookshelf]

I need to export a PNG with 16 bit depth of the final image. The lowest I can get is 24 bit depth when exporting with 8bpc depth. I somehow managed to export a 16 bit PNG earlier but I can't remember how I did this. I've tried looking for an answer online but couldn't find one. I am using greyscale image mode with 16bit integer precision. Does anyone know how to export in 16 bit depth - maybe some kind of plugin is needed?