Pre-heat oven to 160°C. Line two trays with baking paper, then use an electric mixer to beat the butter until pale and creamy add the flour icing sugar and custard powder and use a wooden spoon to stir to combine￼￼.
Use your hands to roll teaspoon of the dough mixture into balls place the balls about 3cm apart on the lined trays use a fork with icing sugar on it, to gently flatten the ball. Bake in preheated oven swapping trays halfway through cooking for 15 minutes or just until the cookie is cooked through and golden brown, removed from oven and set aside 30 minutes to cool.￼￼
Use an electric mixer to beat the extra butter and extra icing sugar in a bowl until light and fluffy add the lemon juice and beat until combined, if the consistency is not right add some milk little at a time. Spread the buttercream over the flat side of one biscuit and sandwich together with another biscuit. Repeat and enjoy