Preheat oven to 180°C.
Cream butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Add mashed feijoas and mix lightly so there are still lumps of it throughout the mixture. In another bowl, stir baking soda into hot milk mix into creamed mixture. Sift in flour and baking powder, fold into the mixture until combined.
Turn into a greased and lined 20cm (approx) round cake tin. Put into preheated oven for 50 min or until cake springs back when lightly touched. Leave in tin for 10 mins before turning out onto a cooling rack.
Mix all the ingredients in a bowl or container until fully mixed. Continue mixing until your desired consistency is reached. Add extra lemon if you are after a more lemony taste. Serve and enjoy!
Note: this recipe turns out even better if mixed in a cake mixer.