1. Preheat oven to 225. Line 2 baking sheets with parchment paper & set aside.
2. Put egg whites and sugar in a heatproof bowl. Heat over a double boiler until sugar has dissolved and egg whites are warm to the touch.
3. Beat egg whites & sugar on high for several minutes, until whites are almost stiff.
4. Add vanilla and beat a minute more.
5. Fold in cocoa powder.
6. Spoon meringue into a pastry bag fitted with a plain tip. Pipe meringue onto baking sheets to make 2 inch domes for the mushroom caps. Pipe a stem shape for each dome. It's a good idea to pipe a few extra stems as some will fall over.
7. Lightly dust with cocoa powder.
8. Bake for about 2 hours, or until they are hard. Store in an airtight container until ready to use.
9. To assemble the mushrooms, melt chocolate over a double boiler. Trim points from stems. Spread chocolate over the underside of a cap and place trimmed end of stem into center of cap. Place mushroom stem side up in an egg carton to harden. Refrigerate until set.