Preheat oven to 180c. Line 2 x 12 hole medium muffin pans with paper cases. In a large bowl, beat butter and brown sugar together using electric beaters until light and creamy. Add eggs, one at a time, beating well.
Sift flour and spice together. Fold into butter mixture with banana and sour cream. Divide mixture between paper cases, filling to three-quarters full. Bake for 20-25 minutes until cooked when tested with a skewer. Cool for 5 minutes. Transfer to a wire rack to cool completely.
Beat extra butter, icing sugar and lemon juice with electric beaters until smooth and creamy. Spread over cooled cakes.