To Make White Chocolate Custard: Heat milk on medium heat in heavy saucepan, stirring, until it comes to a boil. Remove from heat. Add vanilla bean to milk, cover, and steep for 10 minutes. Remove vanilla bean. Meanwhile, whip yolks with sugar until thick and light colored. Whisk in flour. Gradually stir hot milk into the yolk mixture, stirring until smooth. Pour into the sauce pan. Bring to a boil over medium heat, whisking constantly, until it thickens. Lower the heat. Continue cooking the pastry cream, whisking constantly, until it softens slightly; this should take about 2 minutes. Remove from heat, and fold in white chocolate. Transfer pastry cream to small bowl, and cover with plastic wrap. Unwrap when cool, and add creme de cacao. This can be made 1 day in advance; cover and refrigerate until needed.