Cut 1 square of white chocolate baking bar in half; reserve half. Finely chop other half of square and remaining baking bar. Set aside. In small saucepan, cook all remaining white chocolate layer ingredients over medium-low heat until butter is melted and mixture is simmering, stirring occasionally. Remove from heat. Stir in chopped white chocolate until smooth. Spoon into crust-lined pan; spread gently. Refrigerate while preparing dark chocolate layer.