Make the dough according to the recipe instructions.
Lightly grease a baking tray, draw and cut out a pear shaped cardboard pattern 7.5 cm/3 in.
in length and 5 cm/2 in at the widest part.
Roll out the biscuit dough on a lightly floured board to a thickness of 5 mml1/4 in.
Using the cardboard pattern as a guide, cut out the dough with a knife, re rolling the trimmings, if necessary, to make eight mice.
Carefully transfer the biscuits to the prepared baking tray, using a palette knife.
Chill for 30 minutes, or longer if you have time.
Bake the biscuits in a moderate oven (180C, 350F, gas 4) for 18-20 minutes, or until lightly browned.
Remove the hot biscuits from the trays and transfer them to a wire rack to cool completely.
While the biscuits are cooling, make the topping.
Blend the custard powder and sugar with enough milk from the 150 ml/1/4 pint to make a smooth paste.
Bring the remaining milk to the boil and quickly stir it into the custard mixture.
Return the custard to the saucepan and cook for a couple of minutes over a low heat, stirring continuously, until very thick.
Remove the custard from the heat and beat in the egg yolks, butter and 100 g/4 oz of the grated chocolate.
Allow to cool completely, then stir in the crushed biscuits.
Pile this mixture on top of the biscuit bases.
Smooth the mixture with a wet palette knife, tapering off at the narrowest part to form a nose.
Place the remaining chocolate and the 2 tablespoons of milk in a basin over a saucepan of hot water.
Heat gently, stirring occasionally, until the chocolate has melted.
Place the mice shapes on a wire rack standing over a clean baking tray and coat them with the chocolate.
Just before the chocolate sets, decorate the mice with liquorice for tails and whiskers, flaked almonds for ears and silver balls for eyes.
Allow to set before serving the biscuits.