Step 1

Preheat oven to 200°. Using an electric mixer on medium-high speed, beat egg whites and salt in a medium bowl until foamy. With motor running, gradually add granulated sugar and beat until stiff, glossy peaks form. Add powdered sugar and vinegar and beat until meringue is very dense (peaks should still be stiff and glossy).

Step 2

Transfer meringue to a piping bag fitted with a medium round tip and pipe 1"-long cones (these will be the mushroom stems) and quarter-size circles (the mushroom caps) onto a parchment-lined baking sheet. Dust mushroom tops with cocoa powder with a fine-mesh sieve and bake, rotating once, until meringues are dry, 1 ½−2 hours.

Step 3

Using a small paring knife, make a small hole in center of flat side of mushroom tops. Workin one at a time, spread a dab of chocolate on flat side of a mushroom top with a offset spatula and fit smaller end of meringue stem into hole, pressing to adhere. Let chocolate harden.

Step 4

Do ahead: Mushrooms can be made 3 days ahead. Store tightly wrapped at room temperature.