Grease and lightly flour a 10-inch tube pan; set aside. Let butter, eggs, and milk stand at room temperature for 30 minutes. In a medium bowl, combine flour, baking powder and salt; set aside.
In a large mixing bowl, beat the butter with an electric mixer on medium to high speed for 30 seconds. Gradually add sugar, 2 tablespoons at a time. Beat about 6 minutes or until the mixture is very light and fluffy, scraping sides of bowl occasionally. Add vanilla. Add eggs, one at a time, beating for 1 minute after each addition. Add banana and chocolate, beating until combined. Add flour mixture and milk alternately to butter mixture, beating after each addition just until combined.
Spoon batter into the prepared pan, spreading evenly. Use a knife to cut a zigzag pattern through batter to break up any large air bubbles.
Bake in a 350 degree F oven for 55 to 65 minutes or until a wooden toothpick inserted in center comes out clean. Cool in pan on a wire rack for 20 minutes. Remove cake from pan. Cool completely on a wire rack. Makes 16 to 20 servings.