Place all purpose flour, pastry flour, buttermilk powder, salt, sugar, baking powder and baking soda in a large bowl. Whisk to combine dry ingredients.
Cut in the shortening and butter to form a coarse, crumbly mixture. (You can also do this in a large food processor - or do it in two half batches - pulsing the processor to cut the fat into the flour.)
Preheat oven to 425°F. Place three cups biscuit mix in a medium sized bowl. In a well in the centre, stir in almost all the ice water. Form a soft dough by briskly stirring with a fork, adding in reserved ice water if required. Gently knead on a lightly floured board about eight times - briefly, do not overwork dough.
Roll or pat out to a thickness of 1/2". Cut into rounds between two and 2 1/2 inches.
Brush generously with melted butter or lightly with cream or milk before baking.
Place on ungreased baking sheet and bake until lightly golden brown, about 10-12 minutes. Serve hot.
These can also be patted into an eight inch square baking pan or a nine inch cast iron pan. Bake until golden, then cut into squares.