Gently bring the milk to a boil in a heavy-based saucepan. Meanwhile, whisk the egg yolks and half the caster sugar together in a large, heatproof bowl until paler in colour, then whisk in the cornflour and salt. Whisk as you pour the hot milk onto the yolks, then return to the pan and cook over a low heat, stirring, until the mixture coats the back of a wooden spoon. Take off the heat and pour half into a bowl, while this is cooling soak the gelatine in cold water until pliable. Squeeze the water out of 2 of the leaves. Stir the chocolate and these gelatine leaves into the pan with the custard mixture until they have melted, then pour onto the base. Chill.