Long Life Biscuit (Cookie) Fillings

Our answer

There are two problems with keeping filled cookies for any length of time. The first is keeping the filling fresh and the second is that the cookies will start to become a little soggy after a few days as they will absorb moisture from the filling.

A buttercream filling, made with butter and icing (confectioners') sugar, will keep at cool room temperature for up to 5 days but we would not recommend keeping it much beyond this as there is a risk of the butter turning rancid. Buttercreams made with egg yolks or egg whites should always be kept in the fridge.

A ganache will keep at cool room temperature for up to 2 days, regardless of what type of milk or cream it is made with. We would recommend making a ganache with at least equal quantities of dark (bittersweet) chocolate and cream so that it is fairly firm and will stick the cookies together. White chocolate ganache is less stable and we would not recommend keeping it out of the fridge for any length of time. Unfortunately condensed milk needs to be refrigerated after the can has been opened so it should not particularly extend the shelf life of the ganache and can also make it a little too sweet.

The one icing that will have a longer shelf life is a glace icing which is a mixture of icing sugar and water, plus flavourings. This should keep for up to 2 weeks but will tend to turn the cookies soft after a while (depending on how much icing you have used). You could also make a royal icing by adding powdered egg white or meringue powder.

As an alternative you could try making and sandwiching the cookies and freezing them. This should work for the buttercream and ganache filled cookies. The cookies will only take 3 to 4 hours to thaw at room temperature.