Whisk together the flour, salt, and baking powder., Add the butter, working it in to make a coarse/crumbly mixture. Leave most of the butter in large, pea-sized pieces., Stir in the sour cream; the dough won't be cohesive. Turn it out onto a floured work surface, and bring it together with a few quick kneads., Pat the dough into a rough log, and roll it into an 8" x 10" rectangle., Dust both sides of the dough with flour, and starting with a shorter end, fold it in three like a business letter., Flip the dough over, give it a 90° turn on your work surface, and roll it into an 8" x 10" rectangle. Fold it in three again., Chill the dough for at least 30 minutes before using. To make pastry, roll into desired size., Freeze dough for prolonged storage, up to 2 months. To use, thaw in the refrigerator overnight.