Combine butter, lemon rind, sugar and vanilla in a mixing bowl and beat until light and fluffy. Scrape sides with a rubber scraper and beat in eggs, one at a time, until thoroughly mixed.
Sift flour, baking powder and salt into a bowl. Peel bananas and mash to a pulp.
Add dry ingredients to egg mixture in 3 portions, alternately with banana pulp and milk. Beat after each addition until smooth. Pour into two
Beat the sugar, salt and butter until mixture is glossy and smooth. Add remaining ingredients and beat until well mixed. Use to frost cake layers.