Cream together butter, cream cheese, and sugar. Add egg and vanilla. Beat with electric mixer until well combined. Add flour, baking powder, baking soda, salt, and cinnamon. Add mashed bananas. Mix until just combined.
In a small bowl, mix cream cheese, egg, flour, sugar, and vanilla until smooth.
Divide batter evenly into 12 muffin cups. Add cream filling to a piping bag. Pipe cream cheese mixture into muffin batter. Top with walnuts.