Measure out and cut your butter and shortening into 1/2in (1.25cm) cubes. Then, put them in the freezer while you prep everything else.
Add the flours and salt to a food processor and pulse 3-4 times until fully mixed. Fill a small bowl roughly 1/3rd of the way with ice; then add water until the ice floats off the bottom of the bowl. Give the ice and water a good mix to make sure it's very cold and set aside. Last, measure out your sour cream so it's ready to go.
Now, take the butter and shortening out of the fridge and add to the food processor. Pulse into the flour mix (3-4 times) until small pea-sized pieces form. Then, add in the sour cream and pulse 1-2 more times. At this point, you can slowly add the ice water a tablespoon (15ml) at a time, pulsing between pours.
As you add water the flour will hydrate and you will see the dough lose its sandy texture and turn yellow. To test if it's ready, take off the top of the processor and pinch some of the dough between your fingers. If it sticks together, you can stop there. If it's still a little sandy and falls apart, add some more water.
Once you have your dough ready, you want to keep it as cold and unworked as possible. Dump everything out onto a large piece of cling wrap and fold it over the dough compressing it and finally sealing it off. No kneading! The more you work the dough, the harder it will be. Rest the dough in the fridge for at least 2 hours, though. It's better overnight.