Mix dry ingredients in bowl, add cold butter and cut it in until it looks like coarse crumbs. Add ice water and mix until dough forms. Place in frig for about 20 min. Roll 3/4 of dough between 2 sheets of parchment paper too fit an 8x8 pan. Peel off top sheet and flip into pan. Pour in blackberries, mix arrowroot flour and sugar sub. Sprinkle on top of blackberries and dot with the butter.
Roll remaining dough to fit top. Bake in 425 oven for about 30-40 min.
