The Custard Cream Heart biscuits sound really good and look very attractive, but what on earth is vegetable shortening please?
Nigella's Custard Cream Hearts (from FEAST) are crisp biscuits that are sandwiched with a vanilla buttercream. The biscuit hearts are made with a combination of butter and vegetable shortening. Vegetable shortening is a white solid fat made from vegetable oils. In the UK it can be found in the same section of the supermarket as butter and margarine and brands include Cookeen, Flora White and Trex. In the US the best-known brand is Crisco and in Australia it is Copha.
Vegetable shortening is used as it helps to make the biscuits very tender. As vegetable shortening is flavourless it is used in combination with butter so that the biscuits have a melt in the mouth texture and a buttery flavour. If you can't find vegetable shortening then you could use all butter instead, but the texture of the biscuits will be slighty firmer and crisper.