Pepperidge Farm makes two types of puff pastry available in most grocery store freezers: shells and sheets. (click here to go to their website to learn more) Ideally, puff pastry shells are what would work perfectly for the Chicken a la King so if you find those, absolutely buy them and follow the directions on the box. But if you are like me and cannot find puff pastry shells in your store and have to travel 90 blocks to only find the puff pastry sheets, go to this website to learn how to make puff pastry shells from the sheets: Puff Pastry How To
I found this on cooksillustrated.com (my new obsession) and because all of the directions are there under "Puff Pastry Cups" there is no need to repeat them. But if you think that puff pastry is fancy and hard to work with, just know that I improvised and used a wine bottle as my rolling pin and a glass top as my cookie cutter. I posted the pictures of my first batch above to show that my cups did not turn out perfect but it did not matter once they were covered with creamy chicken goodness!