Microwave semi-sweet chocolate and 3/4 cup milk in medium microwaveable bowl on HIGH 2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted.
Add nuts and vanilla; mix well.
Spread onto bottom of foil-lined 8-inch square pan.
Microwave white chocolate and remaining milk in medium microwaveable bowl on HIGH 1-1/2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted.
Spread over semi-sweet chocolate layer. Refrigerate 2 hours.