Yield: 32 mushrooms
3 egg whites
¼ teaspoon cream of tartar
½ teaspoon vanilla extract
½ cup sugar
1 cup reserved Chocolate Buttercream Frosting
Cocoa for dusting 1. Preheat oven to 200°. Line 2 baking sheets with parchment paper. Set aside.
2. In a large bowl, combine egg whites, cream of tartar, and vanilla extract. Beat at medium-high speed with an electric mixer until frothy. Add sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form, approximately 3 minutes.
3. Place mixture in a pastry bag fitted with a large round tip. Pipe approximately 32 mounds (about 1¼ inches wide) to resemble mushroom caps onto 1 prepared baking sheet. Pipe approximately 32 columns (about 1 inch tall) to resemble stems onto remaining prepared baking sheet. Bake for 1½ hours; turn oven off. Allow meringues to stand in closed oven for 2 hours longer.
4. Using a paring knife, cut a small hole in the bottom of each cap. Spread a small amount of Chocolate Buttercream Frosting on cap, over hole. Press stem into hole in cap. Dust with cocoa. Repeat with remaining caps, stems, and frosting.