Beat butter and sugar, in large bowl with the mixer on high speed until light and fluffy. Mix flour and cornstarch in a small bowl. Gradually add the dry mixture, beating well after each addition. Chop chocolate into small pieces. Stir in chocolate.
Drop by tablespoonfuls, onto baking sheets. Bake 20 to 25 minutes or until lightly browned. Makes about 4 dozen.
Let them cool completely and then sprinkle with icing sugar.