Is there an alternative way to make the Custard Cream Hearts if you don’t have a food processor?
Nigella's Custard Cream Hearts (from FEAST) are shortbread-type sandwich cookies that are filled with a vanilla buttercream. It is possible to make them without a food processor - you can instead use a stand mixer or a bowl and wooden spoon. To make the cookie dough, sift the flour, custard powder and baking powder into a bowl. Use a stand mixer with the paddle attachment, or your fingers, to rub the butter and vegetable shortening into the flour mixture, until it looks like breadcrumbs. Stir in the sugar. Whisk the egg and milk together and stir enough of this liquid in to make a soft but not sticky dough (use a wooden spoon or low speed on a mixer). Form into a ball, wrap and chill, then roll, cut and bake following the recipe instructions.
For the custard buttercream, sift the icing (confectioners') sugar and custard powder together. In a separate bowl beat the butter until soft then beat in the icing sugar mixture a little at a time until you have a smooth buttercream. Finally beat in the teaspoon of boiling water and use the buttercream to sandwich the cookie hearts together.