Put the caster sugar, cocoa powder and plain flour in a pan, make a well in the centre and crack in the egg and 2 egg yolks. Stir to form a paste, then gradually stir in the milk, the chopped stem ginger and ground ginger and stir it in.
Put the pan on a low heat and cook for 15 minutes, stirring until it reaches the consistency of thick double cream (keep the heat low or the eggs will scramble). Remove from the heat, then beat until creamy using an electric hand whisk. Divide the mixture between 8 x 50ml (2fl oz) pots or espresso cups, and top each with a piece chopped stem ginger. Chill for 1 hour before serving.