Take the butter out of the fridge. Cut it into squares. Mix the flour, erythritol, baking powder, cinnamon, salt and barberry (I used less than half a bag of berries for pilaf). Throw the butter in there and work it intensely with a fork or your hands. Here's a tip: if you do everything very, very quickly, before the butter gets all soft, you can immediately throw the dough on baking paper, cover with a second sheet, roll it with a rolling pin and cut up cookies. If, on the other hand, the dough is starting to melt and shiny, you will have to put it in the freezer for a few minutes and then roll it out and cut it up