In a medium mixing bowl, beat the butter and cream cheese with an electric mixer on medium speed until combined. Stir in flour. With floured fingers, press a slightly rounded teaspoon of pastry evenly into the bottom and up the side of each of 24 ungreased 1-3/4-inch muffin cups. Bake in a 325 degrees F oven for 20 to 25 minutes or until pastry is golden. Cool 5 minutes in pans. Remove from pans. Carefully transfer to a wire rack; cool completely.
In a small saucepan, bring the 2/3 cup whipping cream just to simmering over medium heat. Remove from heat. Add chocolates; let stand for 2 minutes. Stir until smooth and melted; transfer to a medium bowl. Cover; chill for 1 hour. Beat chocolate mixture with an electric mixer on medium speed until soft peaks form (tips curl). Spoon filling into baked pastry cups.
Cover; chill until serving time. If desired, just before serving, top each tart with whipped cream and a raspberry; lightly sprinkle with powdered sugar.