Preheat oven to 160°C. Grease & line 2 oven trays with baking paper. Use an electric mixer to beat 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 teaspoonful of the dough mixture into balls. Place the balls 3cm apart on the lined trays. Use a fork dusted in icing sugar to gently flatten. Bake in preheated oven, swapping trays halfway through cooking, for 15 minutes or until just cooked through. Remove from oven and set aside for 30 minutes to cool.
Use an electric mixer to beat the extra butter and icing sugar in a bowl until light and fluffy. Add the lemon juice and beat until combined. Spread the butter mixture over the flat side of half the biscuits and sandwich together with remaining biscuits