Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, sugar, baking powder, baking soda, salt, and cinnamon in a large bowl, stirring with a whisk.
Combine banana, milk, and egg, stirring until well blended. Add banana mixture to flour mixture, stirring just until moist. Gently fold in half of chocolate. Spoon batter into a 9-inch round cake pan coated with cooking spray. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 5 minutes on a wire rack; remove from pan. Cool completely.
Combine remaining chocolate and hot water in a small microwave-safe bowl; microwave at high 30 seconds or until almost melted. Stir until smooth. Drizzle glaze over cooled cake.