Combine the flour, Rumford Baking Powder, baking soda, and salt in a food processor. Pulse several times to mix. Sprinkle the butter over top. Process in 30 1-second on/off pulses, or just until the butter is cut in with some bits the size of peas remaining. Drop dollops of sour cream over the top. Process in 15 to 20 1-second pulses, or just until the sour cream is thoroughly incorporated; stop halfway through and stir to lift up the crumbs from the bottom. Check the consistency frequently by stopping and pressing the mixture together with the fingers until it forms a smooth dough. (If it is too dry, add up to 1 tablespoon cold water and pulse until the mixture just holds together.)
Remove the blade; turn out the dough onto wax paper. Press and smooth it into a rough rectangle. Place it between two long sheets of baking parchment or wax paper. Roll it out until evenly thick and roughly 9 1/2- by 13 1/2-inches. Peel off and discard one sheet. With a large knife, trim off and discard the uneven dough edges, then use them to patch the rectangle to achieve the 9- by 13-inch dimensions.