Preheat oven to 350F. Grease a 15x10x1-inch baking pan; set aside.
In a large mixing bowl, beat butter with an electric mixer on medium speed for 30 seconds. Add sugar, baking soda, and salt; beat until combined. Beat in banana, eggs, sour cream, and vanilla until combined. Beat or stir in flour until combined. Spread in prepared pan.
Bake for 25 to 30 minutes or until top springs back when lightly touched and a wooden toothpick inserted near the center comes out clean. Cool in pan on a wire rack.
Prepare Cream Cheese Frosting. Spread over top. Cut into bars. Cover and store in refrigerator. Decorate as desired with Halloween candies before serving.
Cream Cheese Frosting
In a large mixing bowl, beat cream cheese, softened, and butter, softened, with an electric mixer on medium speed until mixture is smooth. Beat in sifted powdered sugar and vanilla until combined. Makes about 2 cups.