Butter a 1 litre pudding basin. Beat the butter and sugar together until it’s light and fluffy. Beat in 1 egg and half the flour, followed by the remaining egg and flour. Beat in the cocoa, a pinch of salt and the milk. Spoon the mixture into the basin and then drop the Lindt balls at intervals around the top, pushing them in down into the pud.
Cover the basin with buttered baking paper and a sheet of foil on top, making a pleat along the top to allow for expansion, then tie the cover on with string or an elastic band.