Preheat the oven to 400°F.
In a large bowl, whisk together the eggs, heavy cream and chives with 1/2 teaspoon kosher salt and 1/4 teaspoon pepper.
Lightly flour your work surface, then unfold the puff pastry sheets. Cut each sheet into 9 squares.
Place a square in each cup of a standard size muffin pan, pressing it firmly into the bottom and sides so that the edges hang over the sides of each cup. (You will have 6 extra squares, which can be frozen again.)
Add 1 tablespoon of your preferred fillings to each cup (ham, cheese, spinach, etc.) then spoon in the egg mixture. (For easy assembly, I transfer the egg mixture into a liquid measuring cup then pour it into each muffin cup.)
Bake the mini quiche until the eggs are set and the puff pastry is golden brown, 15 to 20 minutes.
Remove the mini quiche from the oven and let them cool for 10 minutes in the pan before tracing around the edges with a sharp knife to pop them out and serve.