Preheat the oven to 230°F. Tear off a piece of parchment the size of your largest cookie sheet. Take an 8 x 2 1/2-inch-high dessert ring and trace 3 circles on your parchment; the circles may touch each other or even overlap because the meringue circles will be smaller than the ring size. If necessary, use 2 cookie sheets. Turn the paper over and place on your cookie sheet. You can also use a Springform pan for the cake.
To make the Meringue: Sift the confectioners’ sugar and cocoa together in a bowl.
In the bowl of a stand mixer, or in mixing bowl with a hand-held mixer, beat the egg whites on medium-high speed until they start to stiffen. Turn the mixer to low and add the granulated sugar gradually, and then beat another minute on high speed until the whites are stiff. Sift the confectioners’ sugar and cocoa a second time into the bowl with the egg whites. Turn your machine to low and beat until just combined.
Fill a pastry bag with a 1/4-inch round tip with the meringue batter. Starting from the center of the circles drawn on the parchment, squeeze out spirals until your circle is about 1/2 an inch smaller than the drawn circle. Do not worry if your circles are not perfect; you can use the back of a spoon to smooth over the “holes.” If you do not have a pastry bag, use a silicone spatula to shape 3 circles of meringue batter.
Use any leftover batter to make any shapes you like to decorate the cake, such as kisses, letters, lines, or hearts. Place in the oven and bake for 2 hours. Turn off the oven and let the meringues remain in the oven another 2 hours so they can dry out.
To make the Mousse: Melt the chocolate either on the stovetop or in the microwave. Heat the 1/4 cup of the whipping cream and the sugar in a small saucepan on medium heat, stirring often, until the sugar melts and the cream starts to boil. You can also heat the cream and sugar in the microwave in a glass measuring cup until the sugar is dissolved, about 1 minute. Don’t worry if the cream starts to looks gloppy.
When the chocolate is melted, whisk in the cream and sugar mixture until smooth. Add the egg yolks one at a time and whisk well.
In a large bowl, with an electric mixer on high speed, beat the remaining 1 3/4 cups whipping cream until it is just whipped. Watch carefully and stop mixing when you see the top of the whipped cream look like ribbons or swirls. Add the whipped cream to the chocolate mixture and mix until well blended.
To assemble the cake, you will need an 8-inch cardboard circle. Take a piece of cardboard, trace the inside of your ring, and then cut out the circle. Line another cookie sheet with parchment, place the ring on top, and place the cardboard circle into the ring. Place a tablespoon of the mousse on the cardboard to help glue the meringue circle in place. Place one of the meringue circles in the ring. Pour a third of the mouse into the ring to cover the meringue. Make sure you get some mousse between the meringue and the ring on the sides. Add the second meringue circle and another 1/3 of the mousse. Repeat with the last circle and more mousse and then use a flat blade metal spatula to smooth the top, reserving any extra mousse in a small bowl stored in the refrigerator to decorate the top, if desired. Place into the freezer for 4 hours or overnight.
Remove from the freezer. To remove the ring, place the cake (with the cardboard bottom) on top of a large can of tomatoes or vegetables. Place boiling water in a small bowl. Take a towel or paper towel, dip it into the hot water, and then rub around the outside of the ring; this will help release the ring from the mousse. Go around the entire ring with the hot towel. Take your hands and gently slide the ring down off the cake. You can also use a blow torch to heat the ring and then slide down the ring. Place the cake back on the parchment-lined cookie sheet.
To decorate the cake, you can use the extra meringues you made or you can melt some chocolate c
hips and drizzle the top of the cake in any design you like. I have also used a potato peeler to shave little chocolate circles and then pressed those into the side of the cake. I like to store the cake in the freezer and then remove it 30 minutes before serving so I can cut perfect slices.