Preheat the over to 400F (200C).
Lightly flour a clean work surface and use a rolling pin to roll out the puff pastry to a rectangle that's about 9" x 12". Use a long knife to cut the puff pastry into 24 squares. that's 3 cuts down the longest direction, and 5 cuts across the shortest direction.
Push each puff pastry square into the mini cupcake pan, so that the edges of the puff pastry, come up the sides forming a little cup to hold the mushroom mixture. If any of your squares are a little too small, just use your rolling pin to roll it out a little more. Divide the mushroom mixture evenly among the cups.
Bake for 25 - 30 minutes, rotating the pan once until the edges of the puff pastry are golden. To remove them, use a knife to pop them out of the pan. Finely chop chives, cilantro, or parsley, and sprinkle on the mushroom puffs for garnish.