Spray a baking sheet with cooking spray, or line with non-stick foil.
Clean mushrooms with a damp paper towel. Carefully break off the stems. Chop the stems extremely fine, discarding the tough end of the stem.
Heat oil in a large skiller over medium heat. Add garlic and chopped mushroom stems to the skiller. Fry unti lany moisture has disappeared, taking care not to burn the garlic. Set aside to cool.
When garlic and mushroom mixture is no longer hot, stir in cream cheese, Parmesan cheese, black pepper, onion powder, and cayenne pepper. Mixture should be very thick.
Using a little spoon, fill each mushroom cap with a generous amount of stuffing. Arrange the mushroom caps on the prepared baking sheet.
Bake for 20 minutes, or unti lthe mushrooms are piping hot and liquid starts to form under the caps.