Nigella's Custard Cream Hearts

From Nige's lovely book 'Feast'...

*For the biscuits:

175g plain flour
3 tbsp custard powder
1 tsp baking powder
50g unsalted butter
50g vegetable shortening
3 tbsp caster sugar
1 egg
1 tbsp milk (or more, to bind)

*Preheat the oven to gas mark 4/180 degrees
*Mix flour, custard powder, baking powder.
*Add the butter and shortening in smallish cubes and mix to a crumbly mixture
*Add the sugar and mix again.
*In a separate bowl, mix the egg and milk, then add this to the main bowl, and mix it until it clumps into a ball. You may not need all the liquid so go slowly, (or you can always add a little more flour, or more milk if it's too dry)
*Form the dough into a ball , press it into a flat disc and wrap in clingfilm. Pop it in the fridge for 20 minutes.
*Flour a clean surface, and roll out the ball to about 4mm thickness. Use a 5cm heart shaped cutter and cut out as many as you can, putting them on a lined baking sheet as you go along, and re-rolling the scrap bits of dough until it's all used up.
*Prick the outside edge of the hearts with a fork or other such pronged device.
*Cook on lined baking sheet for 15 minutes, or until golden.
*Cool on a wire rack.

*For the custard cream:

1 tbsp custard powder
100g icing sugar
50g soft unsalted butter
1 tsp boiling water

*Mix the custard powder and the icing sugar. Add the butter and mix until you get a smooth cream. Add the teaspoon of boiling water and combine again.
*Sandwich each biscuit with about one teaspoon of the cream. Gently spread a layer of the cream on an unpricked side (once the biscuits are cool), then wiggle a matching heart shape on top of it. I found there was enough cream to be quite generous with the filling, and that it made about 20 finished biscuits altogether.

*I thought that half a teaspoon of raspberry jam on top of the custard cream filling would have made a rather delightful addition, also next time I will add a tablespoon of rosewater to the cream filling.