Combine first 5 ingredients; beat at high speed of an electric mixer until foamy.
Add sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves (2 to 4 minutes).
Spoon mixture into a decorating bag fitted with a large round tip. Pipe 32 (1 1/4"-wide) mounds to resemble mushroom caps and 32 (1"-tall) columns to resemble stems onto a parchment paper-lined baking sheet.
Bake at 200 for 1 hour and 30 minutes; turn oven off.
Let meringues stand in closed oven 2 hours.
Spread a thin layer of melted chocolate on flat side of caps. Trim rounded end of stems to make flat; press against chocolate to attach stems to caps.
