Preheat oven to 315ºF. Lightly spray 12-cup Bundt pan with vegetable oil baking spray; set aside. For cake: Reserve 2 tablespoons of frosted walnuts. In large bowl, combine remaining package of walnuts, flour, baking powder, salt and cinnamon; set aside.
Place butter and chocolate chips in 2-quart microwave-safe bowl. Microwave on HIGH 1 1/2 to 2 minutes stirring every 30 seconds until chocolate is just melted. Transfer butter mixture to large bowl of electric mixer. Stir in granulated sugar. Add eggs, one at a time, mixing thoroughly after each addition. Add coffee and rum; mix well. Add flour mixture, 1/2 cup at a time, mixing well after each addition.
Pour cake mixture in pan. Bake 1 hour 20 minutes to 1 hour 30 minutes or until toothpick inserted in center of cake is clean when removed. Cool cake in pan 15 minutes. Gently loosen edges of cake; invert onto cooling rack. Cool completely.
For icing: Combine powdered sugar and meringue powder. Gradually add water, 1 tablespoon at a time, mixing with electric mixer until desired consistency is achieved. Drizzle cake with icing; sprinkle remaining 2 tablespoons walnuts over top.
