Reheat oven to 350F. Grease 6 cups of a muffin tin with cooking spray.
In a large bowl, mix together flour, sugar, baking powder, pumpkin pie spice, and cinnamon. Add in remaining muffin ingredients. Fold everything together until just combined.
Evenly distribute batter among the 6 cups, smoothing the tops as you go.
In a separate bowl, whisk together cream cheese filling ingredients until smooth and creamy. Add mixture to a sandwich-sized plastic bag. Push contents to the corner of one side and seal the bag. Cut the tiniest corner of the tip of the bag with scissors so that the filling can slowly funnel out.
Push cut corner of the plastic bag into the center of each muffin batter and fill insides with the cream cheese mixture. Evenly distribute cream cheese mixture along with all the muffins, leaving a dollop on top of each.
If using walnuts, chop finely and sprinkle around the edges of the pumpkin spice muffin mixture.
Bake at 350F for 18-24 minutes, or until a toothpick stuck into the muffin part comes out clean and cream cheese mixture no longer jiggles.
Muffins will taste best when cooled and refrigerated for at least an hour, then brought to room temperature or reheated in the microwave. As with regular cheesecake, the filling tastes much better as it cools.