Whisk together the flour, salt, and baking powder.

Coarsely grate the butter onto the flour then use your fingers to work it in a bit. 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. This recipe makes enough for a rectangle approximately 12" x 24".

To make ahead: Freeze dough for prolonged storage, up to 2 months. To use, thaw in the refrigerator overnight.