Cut the soft white part in dices then place them in a saucepan and pour the milk and whipping cream over them. Let the bread dices soak in the milk and cream mixture for at least 1 hr, or more if possible, in the refrigerator.
Put the saucepan on medium heat and start stirring with a wooden spoon the bread and milk mixture. Once the milk starts heating, add the water-dissolved cornstarch. Continue stirring until the mixture thickens.
Once done, pour in a bowl and leave it to cool. Cover the bowl with a plastic wrap and place the cream in the refrigerator in order to thicken before using it.