In a very large bowl combine flour, sugar, salt and baking powder. Cut in butter leaving pea sized chunks.
In a 2 cup glass measure whisk water, sour cream, and vinegar; pour, all at once into flour mixture, and quickly distribute. Stop!
Make sure dough is slightly crumbly. Divide dough mixture into 3 equal portions (about 12 to 13 ounces each) Refrigerate for 2 hours and best overnight (finish by hand). DO NOT OVER MIX- the finished dough should break, not stretch.
Gently knead in bag to bring dough together. Form into discs.
Use at once or wrap and store in refrigerator for several days or in freezer for up to 1 month. Thaw overnight in refrigerator if frozen.