Preheat the slow cooker if necessary; see the manufacturer's instructions. Pour the milk into a saucepan and bring just to the boil. Remove from the heat, add the chocolate pieces and instant coffee and set aside for 5 minutes, stirring occasionally, until the chocolate has melted.
Put the whole eggs, egg yolks, sugar and vanilla extract in a bowl and whisk until just mixed. Gradually whisk in the hot chocolate milk until smooth. Strain through a sieve into the pan, then pour into 4 tall heatproof mugs, each 250 ml (8 fl oz), checking first that they will fit in the slow cooker pot.
Cover the tops of the mugs with foil and stand them in the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the mugs. Cover with the lid and cook on low for 3–3½ hours or until the custards are set and the tops can be lightly pressed with a fingertip.
Lift the mugs carefully out of the slow cooker pot using a tea towel. Leave to cool, then transfer to the refrigerator for at least 4 hours until chilled. Just before serving, whip the cream with the sugar and vanilla until soft swirls form. Spoon the topping over the custards and lightly dust with cocoa powder. Serve with dainty biscuits.