To make the biscuit batter, preheat the oven to 180C/Fan160C/Gas Mark 4. Line 2 baking trays with baking parchment.
Beat the butter and sugar in a large mixing bowl, with an electric hand held beater until light and fluffy, this will take 3-5 minutes. Fold in the flour, baking powder and cornflour.
Fill a piping bag fitted with a 1cm star nozzle with the batter. Fold the top of the bag. Pipe 8 x 5cm-6cm circles on each baking tray, allow space between each biscuit as they will spread during cooking. Bake for 15-18 minutes. Swap the trays around half way during the cooking time.
Cool on the baking tray then transfer to a cooling rack.
Meanwhile, to make the butter cream, place the butter, milk and half the icing sugar in a large bowl and beat with a large metal spoon until smooth. Mix in the remaining icing sugar.
Divide the butter icing between 4 separate bowls. Colour each one to your colour liking. Spoon one of the colours into a piping bag fitted with a 1cm star nozzle. Fold the top of the bag. Pipe swirls of icing on the flat side of a biscuit. Sandwich together with a flat sided non iced biscuit. Repeat with a second biscuit. Repeat the above process with the remaining butter icings.