In 1-quart heavy saucepan, heat whipping cream over medium heat just until cream comes to a simmer. Remove from heat; stir in chocolate until melted. Stir in butter pieces, a few at a time, until melted. Refrigerate until thickened.
Heat oven to 200°F. Line cookie sheet with cooking parchment paper. In medium bowl, beat egg whites with electric mixer on medium speed until soft peaks form. Gradually add 1/3 cup granulated sugar, beating on high speed just until stiff peaks form. Fold in 1/3 cup powdered sugar.
In small bowl, mix 1/4 cup powdered sugar and the cocoa. Fold cocoa mixture, 1/3 at a time, into beaten egg whites. Spoon mixture into decorating bag fitted with star tip. Pipe into 24 (1 1/2-inch) rounds, about 1 inch apart, on parchment paper.
Bake 1 hour to 1 hour 15 minutes or until crisp. Slide parchment paper with cookies onto cooling rack. Cool completely, about 5 minutes.
For each sandwich cookie, spread chilled ganache on flat side of 1 meringue; top with second meringue. Meringue becomes tough if it absorbs moisture; store in a loosely covered container at room temperature. If the humidity is high, transfer to a tightly covered container.