Using a food processor or a pastry blender cut cold butter into flour and salt until mixture resembles a coarse meal. Small pieces of butter should still be visible.
Pour sour cream and cold water over the mixture and work in by tossing with a fork until dough begins to form. Use your hands as little as possible and work the dough as little as possible.
Divide dough into 2 balls, flatten into 2 rounds, wrap in plastic wrap and place in the refrigerator to rest for a minimum of 20 minutes. You can make your dough the previous day but make sure you take it out of the fridge for 10 minutes to warm slightly before rolling out.
You will need only one of the pastry rounds for this recipe. The second can be easily frozen for another time.
Roll the pastry out into a 9 inch circle and place in an 8-9 inch fluted edge tart pan