Preheat the slow cooker if necessary; see the manufacturer's instructions. Break the 125 g (4 oz) chocolate into pieces, put into a saucepan with the butter and heat gently, stirring occasionally, until melted. Take off the heat and set aside.
Whisk together the whole eggs, egg yolks, sugar and vanilla extract in a large bowl with an electric whisk for 3–4 minutes or until light and frothy. Gradually whisk in the melted chocolate mixture.
Sift the flour into the chocolate mix and fold together. Pour into 4 buttered and base-lined individual metal pudding moulds, each 250 ml (8 fl oz). Press 2 squares of chocolate into the centre of each, then loosely cover the tops with squares of buttered foil.
Transfer the pudding moulds to the slow cooker pot and pour boiling water into the pot to come halfway up the sides of the moulds. Cover with the lid and cook on high for 1¼–1½ hours until well risen and the tops spring back when lightly pressed.
Loosen the puddings with a knife, turn out into shallow serving dishes and remove the lining paper. Sprinkle with sifted icing sugar. Serve with marshmallows, spoonfuls of vanilla ice cream or crème fraîche.