Beat first 5 ingredients at high speed with 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-inch-wide) mounds for mushroom caps and 32 (1-inch-tall) columns for 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 to caps. Sprinkle with cocoa.