Preheat oven to 325°F. Grease and flour 12-cup bundt style baking pan; set aside.
Combine sugar and butter in large bowl. With a mixer, beat on medium speed until creamy. Add buttermilk, eggs and rum flavoring; continue beating until well mixed. Reduce speed to low; add flour, soda and salt. Beat, scraping sides of bowl often, until well mixed. Add mashed bananas and coconut; continue beating until well mixed.
Pour batter into prepared cake pan. Bake for 60-75 minutes or until long toothpick inserted into cake comes out clean. Cool in pan for 5 minutes. Invert onto cooling rack, remove from pan. Cool for 1 hour.
Glaze: Place powdered sugar and vanilla into small bowl. Add milk slowly while stirring to make mixture desired consistency. Place cooled cake on plate and drizzle with glaze.