Cream the butter and cream cheese together. Gradually add the white sugar, and continue beating until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in the mashed bananas and vanilla. Add flour, baking powder, and soda; mix until batter is just moist.
In a small bowl, mix together chopped pecans, brown sugar, and cinnamon.
Divide 1/2 batter into two greased and floured 8 X 4 inch loaf pans. Sprinkle pecan mixture over batter in pans and top with butter.
Bake at 350 degrees F for 45 minutes, or until an inserted toothpick comes out clean.