1.Preheat oven to 180°C. Line eight holes of a 12-hole muffin pan with paper cases.
2.Beat butter, extract, sugar, eggs, sifted flour and milk in a small bowl with electric mixer on low speed until ingredients are combined. Increase speed to medium and beat until mixture has changed to a paler colour. Drop ¼ cups of mixture into paper cases.
3.Bake about 20 minutes. Stand cakes in pan 5 minutes before turning, top-side up, onto wire rack to cool.
4.To make butter cream, beat butter in small bowl with electric mixer until as white as possible; beat in sifted icing sugar and milk, in two batches.
5.Use scissors to cut marshmallows into small pieces. Spread cold cakes thickly with butter cream; top with marshmallow pieces.