If you’re in the UK/Europe, the best alternative I’ve found are Bahlsen’s Milk Dark Chocolate Leibniz.

Still has the issue of what is this branded product really though.

I think another option is chocolate 'malted milk' (in the UK) - depending on your preference for ratio of biscuit to chocolate to marshmallow. Leibniz will have more/thicker chocolate, but malted milk will break a bit easier in the mouth (softer/crumblier biscuit).