Bake tart cases according to pack directions. Leave on oven tray and set aside to cool.
Place custard, sugar, butter and salt into a medium saucepan. Place over a medium heat. Simmer, stirring constantly for 5 minutes or until very thick. Immediately divide the mixture between the 12 tart cases. Refrigerate for 1 hour or until cool.
Place chocolate in a heatproof bowl and place over a saucepan of simmering water (ensure that the water does not touch the base of the bowl). Stir occasionally until melted. Spoon into a small plastic zip lock bag. Snip off one point with scissors to form a piping bag. Drizzle chocolate onto tarts. Refrigerate until set. Store in the refrigerator.