After eating them for a week, I can report that hard tacks with butter are almost indistinguishable from the ones I made with olive oil. If you would want to try them yourself, then you might want to up the heat for the last hour to about 140 °C (normally you want to make them at 110 - 120 °C). Mine came out slightly moist even after seven hours of baking. I don't know why there's this problem, though. There shouldn't be that much more moisture added with butter to make this difference. I'll make a few more batches to see if the effect is repeatable.
Alternatively, leave them to dry in cold air for a day or two. I know that you have some pun about 'hot air', stop that. Cold air holds less moisture and is much more efficient. :P
Recipe for hard tacks since I forgot about it the last time:
- 1.5 kilogramme of flour.
- 30 grammes of butter (or three tablespoons of olive oil).
- 2 - 3 tablespoons of salt.
You have to mix together with water. Add as little as you need, the moment the dough will absorb all of the other ingredients without sticking too much to the bowl or fingers, it's ready. Make a slab out of it that's roughly 1 centimetre thick, cut it into squares. Now we have a tedious/fun part! Take a toothpick, break it in half and with the help of those two sticks use their wider ends to make a bunch of holes in each of the squares. You want about 1 hole per cm². That's to allow for a quicker escape of moisture.
Drop it into the pre-heated to about 110 °C oven and go do something else. For the next six or seven hours, you will only have to flip them every hour or so and perhaps rotate the 'shelves' in the oven (is there a term for them?) if you needed more than one. Get a book or something. For the last hour, you might like to apply the above advice about bringing the heat to about 140 °C, though.
Oh, and feel free to convert the proper units to the wizard ones at your own leisure. :P