Preheat your oven to 200 degrees Celsius and line two trays with baking paper.
In a large bowl place your very soft butter (If you've baked these before, you'll notice I've changed this recently to soft not cold butter, it makes them easier to roll). Using hand held beaters (or a stand mixer) beat on medium speed until the butter and sugar become smooth and lighten in colour. Turn beaters up to high and and beat until they form a whipped, fluffy and light mixture.
Sift over the custard powder and plain flour and using a spoon mix together. Don't worry if the mixture looks too dry at this point, it will come together I promise.
Once the mixture is starting to clump together you can get your hands in there and start squishing together to form a solid dough like texture.
Take a teaspoon of cookie dough and squish into a rough ball shape before rounding off slightly by rolling between your palms. Place onto your prepared sheets a few centimeters apart. If you'd like truly uniform balls weigh them out as you go, I make mine approx 13grams per ball.
Once all your balls are rolled, take a lightly floured fork and press down on each ball once.
Pop straight into a hot oven for 10 minutes. These cookies will not change colour on top, so to check lift one up and the bottoms will be a light brown.
Remove and cool for 5 minutes on the tray before transferring to a wire rack to cool fully. Once your cookies have cooled down you can start work on your lemon icing to sandwich in between.