To make the crust: In the bowl of a food processor, combine the butter, cream cheese, flour and salt. Pulse to combine. Pulse, pulse, pulse until the ingredients start to come together and resembles Parmesan. Add the egg yolk and 1 tablespoon of water. Pulse a couple of more times until the ingredients start to form a ball. If the mixture is exceedingly wet, add some more flour. Or conversely, if the mix is dry, dry, dry, add in a few more drops of water.