Preheat oven to 180°C. Grease base and sides of a 7cm deep, 19cm square cake pan.Using an electric mixer, beat butter, sugar and vanilla until pale. Add the eggs, beating well after each. Combine together with sour cream and bananas. Sift the flour and bicarb of soda over batter and gently fold through. Spoon batter into cake pan. Smooth surface. Bake for 50 to 60 minutes. Let cake stand for 10 minutes before transferring to a wire cooling rack.To make icing; combine everything together with electric beaters. Wait until cake has completely cooled before icing/frosting.