My Personal Touches
I love recipes that are hard to do badly and reward you for creativity. I've linked to my base, gold standard, never fail version. I just never make it exactly that way.
I omit the nuts and the coconut and I add more raisins. That one change makes it a home run every time - and it's close to the version found in my local bakery.
Things I've tried
Raisins work best for my audience, about five per tart.
Almost any dried fruit works; I've tried dried apple chips and sweetened dried cranberries. Thin dried apples (with raisins) were perhaps the best I've ever had. Dried cranberries instead of raisens were also very good - but it was a cranberry tart, not a butter tart.
I tried cornflakes to add a slightly crunchy texture without using nuts or coconut. It was differently good, but we liked a smooth texture with plumped raisins better.
My best batch had a single rum-filled chocolate gently pressed into the tart filling before baking. It melted and oozed into the filling. I've been trying to find more of those ever since.
I sometimes use soft margarine instead of butter in the crust. I don't have a stand mixer and this makes life a bit easier. They aren't quite the same - but it's still delicious. If you don't have butter on hand, it's still worth the trouble making them.
Cocoa, maple syrup, almond flavoring and cinnamon are all worth a try.
Use a proper tart pan. A cupcake/muffin pan throws off the crust to filling ratio and it becomes overwhelmingly sweet. (You could fill the cups half way, producing a wide, shallow tart.)
Apply baker's spray liberally, unless you are using silicone bakeware. The filling sometimes bubbles over and it's very sticky.
Loosen the cooled tarts with a plastic knife or a tiny silicone spatula; it's less likely to break the tart.