These funny little confections are a traditional garnish for Buche de Noël. My mother used to make them with us when we were growing up -- they're a great project to make with young kids. They also make for a fun edible present. —Merrill Stubbs
To make the meringues, first heat the oven to 225 degrees and line 2 baking sheets with parchment or silicone.
In the bowl of a standing mixer, beat the egg whites on low speed until foamy, about a minute. Add the cream of tartar and beat on medium speed until the whites form soft peaks, another minute or two.
Increase the mixer speed to medium-high and add the sugar gradually until you have glossy, stiff peaks. This should take about 5 minutes.
Fit a pastry bag with a 1 1/2-inch round tip and spoon about half the meringue into the bag. Twist the top and pipe 1 1/2-inch rounds onto one of the lined baking sheets, leaving an inch or so between the rounds.
Add the rest of the meringue to the bag and using the second baking sheet, Then pipe the mushroom stems, starting a little thicker at the base and then thinning out as you work up, creating a cone shape. (Count as you get towards the end of the meringue and make sure you have an even number of caps and stems -- if not, pipe more of whatever you need.)
Dip your finger in a little water and gently smooth the tops of the little rounds. (No need to worry about the stems, as you'll be trimming them after they're baked.) Bake the meringues for 1 1/2-2 hours, until crisp and dry -- do not let them brown. Let them cool on the baking sheets.
In the meantime, melt the chocolate in the microwave or a double boiler and whisk in the spices if using. Let the chocolate cool a little so that it thickens up a little and isn't so runny.
While the chocolate is cooling, use a serrated knife to carefully trim about 1/4 of an inch off the the pointed tips of the mushroom stems, creating a flat plane.
Dip the bottom of each meringue mushroom cap into the chocolate, creating a thin layer, and then attach the the narrow end of the stem, using the chocolate as glue. Arrange the mushrooms right-side-up on a baking sheet and refrigerate until the chocolate sets. Dust the tops of the mushrooms with cocoa powder to finish.