Combine flour and salt in the work bowl of a food processor and pulse several times to mix. Cut butter into 15 to 20 pieces and add to work bowl. Pulse repeatedly, about 20 times in all, to reduce the mixture to a fine powder. Open the cover and check the consistency occasionally to avoid overprocessing and turning the mixture into a paste. Add sour cream to work bowl and pulse 5 or 6 times more, or until the dough forms a coherent ball. Don't overprocess. Remove dough from work bowl to a floured surface and press it into two 15 cm disks. Wrap in plastic and refrigerate. Keep the dough in the refrigerator for up to two days, or doublewrap in plastic and freeze for up to several weeks.