In the bowl of a stand mixer, add the aquafaba, toasted sugar, and cream of tartar. Let it whip at medium speed, then when it starts to stiffen and rise in volume, raise the speed to high until it reaches stiff peaks.
Add the salt and vanilla
Move two oven racks closer tot he middle of the oven and preheat to 225°F
Line two baking trays with either parchment paper or a silicone baking mat. Using a piping bag with a round tip, pipe the mushroom caps by squeezing at the base and releasing the pressure as you come up. Dip your fingers in cold water to smooth down any pointed tops.
In the other tray, make the stems by making smaller circles. It will flatten out as it dries, but just make it smaller than the caps.
Bake until dry to the touch and can pull away from the parchment paper. Remember, they are still soft when they are warm, they get firmer as they cool. This drying out process can take 3 hours
Shut off oven and allow to cool inside until they reach room temperature, this could take another 1-3 hours.
Temper your dark chocolate.
Color the caps with cocoa powder. Dip the stems with the chocolate and sick the cap on top. Lay on a plate to allow it time to harden. Repeat the process until all are attached.