Preheat your oven to 400 degrees F. Spray the muffin tin cups with nonstick cooking spray. Set aside.
Unfold the puff pastry sheets. Cut each sheet into 9 equal squares (like a hashtag #) to make a total of 18 pieces. Place each dough square into a muffin tin cup, and press down slightly to mold it to the cup.
Add about 1/2 tablespoon of pizza sauce to each cup - enough to cover the bottom of each pastry. Next, add 1 tablespoon of shredded cheese to each cup. Then, add about 5-6 pepperoni slices on top of the cheese before adding even more cheese on top.
Bake the pizza puffs for 20 minutes unilt the cheese is melted and the pastry is golden brown. Remove from oven and let cool for about 5 minutes before serving.