- 2 large egg whites, room temperature
- 8 tablespoons superfine sugar
- 1/4 teaspoon vanilla extract
- Powdered sugar (for garnish)
- Natural unsweetened cocoa powder (for garnish)
Position rack in center of oven and preheat to 250°F. Line large rimmed baking sheet with parchment paper. Using electric mixer, beat egg whites and cream of tartar in medium bowl until stiff peaks form. Add superfine sugar 1 tablespoon at a time, beating well after each addition until dissolved and mixture is stiff and glossy. Beat in vanilla.
Transfer mixture to large pastry bag fitted with 1/2-inch plain pastry tip. To form mushroom caps, pipe twelve 1 1/2-inch-diameter mounds onto prepared baking sheet. To form stems for mushroom caps, on same baking sheet position pastry bag and tip 1/4 inch above surface of sheet and pipe out 12 steady pointed cones of 1- to 1 1/4-inch-high meringue, pulling straight up while squeezing pastry bag. Reserve some meringue in pastry bag for attaching stems to mushroom caps.
Bake meringues until dry to touch and still white, 60 to 65 minutes. Remove from oven and let meringues cool on baking sheet. DO AHEAD Can be made 1 day ahead. Store meringues in airtight container in single layer at room temperature. Wrap pastry bag with remaining meringue in plastic and chill.
Carefully poke small hole in underside of each meringue mushroom cap; pipe in small amount of meringue, then carefully insert stem. Repeat with all remaining caps and stems. Dust mushrooms with powdered sugar, then lightly with cocoa powder. Lean meringue mushrooms against yule log and serve.