Preheat oven to 350 degrees F.
Lightly spray two 12-cup muffin tins with cooking spray. (You can use paper muffin cups to line the muffin pans instead.)
Beat softened butter with a heavy duty mixer until creamy.
Add brown sugar and beat until light and fluffy.
Add eggs, 1 at a time, beating well after each addition.
Add sour cream and vanilla and blend well.
In a separate bowl, blend together flour, cinnamon, nutmeg, baking soda, baking powder, and salt.
Gradually add flour mixture to butter, brown sugar and sour cream mixture. Beat until just combined. Do not over beat.
Stir in mashed bananas and nuts.
Gently spoon batter into muffin tins, filling ¾ full.
Bake at 350 degrees for 25 to 30 minutes or until wooden tooth pick inserted in center of one muffin comes out clean.
Cool in the pans for 15 minutes, then remove and cool completely on wire racks.
Make a small hole in the top of each muffin, by using the handle of a wooden spoon.
Spoon the cream cheese filling. Into a pastry bag with a star tip, or a zip top bag.
If using a zipper bag, snip the tip off of one corner.
Insert tip of the bag into the center of each muffin and squeeze our a generous amount of filling. (Alternatively, muffins can be frosted on top with the Filling.)
