First, preheat oven to 400°F and spray a muffin tin with cooking spray.
On a floured surface, roll out one puff pastry sheet until it's around 1/4 inch thick and slice in straight lines to create 12 (mostly) equal pieces.
Press each piece of dough into a muffin cup.
Melt butter in a small bowl and stir in garlic powder and parsley.
Brush each cup with butter mixture and set aside.
Then, in a medium bowl, combine the room temperature cream cheese, spinach, mozzarella cheese, parmesan cheese, salt, and pepper.
Note: if thawing frozen spinach, make sure to heat it in the microwave according to the package instructions (usually 2-3 minutes) and thoroughly drain to remove all excess liquid. I also like to dab it with a paper towel before adding to the cream cheese mixture. You can also use baby spinach if you prefer fresh. Be sure to chop well.
Once combined, spoon the spinach mixture into the cups until they're around halfway full.
Optional: Create and egg wash by whisking one egg and brushing it on the exposed puff pastry dough. This will add a lovely glossy look to that golden brown color when done.
Bake in the preheated oven for 10 to 15 minutes, until the puff pastry is golden brown.