Fillings have dual purpose: to add a dash of colour and flavour partway through the cake, and importantly in our case to glue pieces together.

When selecting a filling make sure you keep both of these purposes in mind.

As a rule of thumb, the lighter the cake, the squishier the filling can be, so fruitcake almost never has a filling, whereas it wouldn't be a swiss roll or pavlova without something sticking the layers together.