In small saucepan, combine white chocolate baking bar and whipping cream; heat over low heat, stirring constantly until baking bar is melted and smooth.
Stir in vanilla. Pour into small bowl; cover with plastic wrap. Refrigerate 4 hours or overnight until mixture is very cold and thickened, stirring occasionally.
Line 8 muffin cups with foil or paper baking cups. In small saucepan, combine milk chocolate and oil; heat over low heat, stirring until chocolate is melted and smooth. Remove from heat.
With small paint brush, pastry brush or teaspoon, brush thin layer of chocolate evenly over sides and bottom of each foil cup. Place in freezer until firm, about 5 minutes. Repeat coating process of cups using remaining chocolate. Place in freezer until firm, 10 to 20 minutes.
Carefully peel foil cups from chocolate shells; place shells in refrigerator while preparing filling. With electric mixer, beat white chocolate mixture at high speed until light and fluffy. DO NOT OVERBEAT.
To serve, spoon 1/2 cup mousse into each chocolate shell. If desired, garnish with stemmed maraschino cherry. Store in refrigerator.