Preheat oven to 180°C/160°C fan forced. Grease and line a 28 cm x 18 cm lamington pan with baking paper.
Cream butter, sugar and vanilla until light and creamy. Beat in eggs, one at a time, beating well after each addition; fold in flour and milk.
Divide mixture in half; stir Dark Melts into one half and White Melts into the other half.
Spoon mixture alternately into prepared pan and bake 35-40 minutes or until a skewer inserted into the centre comes out clean. Cool. Spread Milk Melts over the top, allow to set before cutting into slices to serve.