1. Preheat oven to 375 degrees F. Line a baking sheet with parchment paper or a silicone baking sheet.
2. Place unsweetened and semisweet chocolate and butter in a medium microwave-safe bowl. Heat at 75 percent power until softened, about 2 minutes. Stir, and continue to microwave until completely melted, about 2 minutes more. Whisk sugar, eggs and vanilla into chocolate mixture until smooth. Sift flour, cocoa, baking powder and salt into another bowl. Gradually whisk dry ingredients into wet ingredients until moistened. Fold batter together using a rubber spatula. Do not over mix.
3. Drop heaping tablespoons batter onto prepared pan, 1 inch apart. Bake until cookies spring back when lightly touched, about 6 minutes. Cool cookies slightly.
4.Transfer half cookies to a rack. Turn remaining cookies on pan over-flat side up. Place a marshmallow on top each flipped cookie; return pan to oven. Cook just until marshmallow begins to soften and puff, about 3 minutes. Cool marshmallow topped cookies slightly, about 2 minutes. Top with remaining cookies, pressing lightly to make sandwiches. Cool whoopee pies completely on wire racks. Serve.
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