In a small saucepan over medium-heat, whisk together the milk, sugar, cornstarch, vanilla and salt until dissolved. Cook, whisking, until the mixture begins to thicken and the milk is hot to the touch, then temper the eggs by very slowly pouring half of the hot mixture into the yolks, whisking continuously to raise the temperature of the yolks without them curdling. Return the mixture to the saucepan and continue to cook, whisking until very thick. Remove from heat, pour the custard into the bowl with the coconut mixture, and whisk until the butter has melted and the custard is smooth.