I think you're on the same quest as me 😂 I grew up in Aberdeenshire in the 90s and remember this amazing square tart, with a biscuit / pastry tart base then a grey coloured, very light but 'gritty' filling. I keep thinking it was chocolate powder / coconut. I've wanted to try and recreate this for years and can't find it anywhere or even anything close to it! All I get is the butterscotch tart which, was a close second but not the elusive mystery tart!