Im making jamtart but can you double the grams of the butter and flour and water to make a bigger one?
I don't see why you can't double the amount, as long as it is everything you double, including the jam. Make sure that it is not rolled out thicker, otherwise cooking time will change.
Liam when you say "make a bigger one" are you talking about making a bigger batch of similar sized tarts or making a really big tart ?
To make more, you can easily just double the whole recipe and make double the amount.
However if you are thinking of doing some sort of giant tart then you may have to adjust oven temperature and cooking times. Thicker pastry would take longer to cook right through and so would burn on the outside before the inside was cooked if left at same temp. It might also make the pastry a bit harder due to the longer cooking times....
just a thought
Steve