Preheat oven to 325°F. Butter a half-sheet pan, line with parchment, and butter parchment.

Sift cocoa, flour, baking soda, and baking powder into a small bowl. Add salt and whisk to combine. Set aside.

In the bowl of a stand mixer fitted with the whisk attachment, combine egg, vanilla, and sugar. Mix on medium-low for 1 minute, then increase the speed to medium and whip for 5 minutes. Scrape down sides of bowl and whip on medium-high for another 5 minutes.

Add mayonnaise and whip in on medium, 1 minute.

Fold in flour mixture and water in two alternating additions.

Pour batter into prepared baking sheet and smooth. Bake 10 to 15 minutes, or until the cake springs back when pressed lightly and a skewer inserted into the center comes out clean.

Let cake cool completely.

Place a sheet of parchment on the back of a second baking sheet. Run a knife around the edges of the cake and invert onto the second baking sheet. Freeze for about 15-20 minutes.

Melt the dark chocolate, either in a double boiler over simmering water, or in the microwave using 20-second bursts.

Cut two 7 1/4 inch disks from the frozen cake and spread each with chocolate. Freeze for 1 hour.

Freeze the bowl and whisk attachment from a stand mixer.

Place heavy cream in the stand mixer and whip until it holds soft peaks. Transfer to another bowl and refrigerate.

Melt the chocolate in a large bowl, either by setting it over simmering water, or by heating it in a microwave in 20-second bursts, stirring between each. Once melted, set aside to cool to 100-120°F.

Place the egg, egg yolks, and sugar in the bowl of a stand mixer and set over a pan of simmering water. Whisk continuously until the mixture thickens and the temperature reaches 183°F. Immediately place the bowl on the stand mixer fitted with the whisk attachment and whip on high until cool, 7 minutes.

Stir one-third of the whipped cream into the melted chocolate. Fold in the egg mixture, then fold in the whipped cream. Transfer to a piping bag fitted with a 1/2 inch plain tip.

Place a silicone baking mat on a baking sheet and place an 8-inch by 1 3/8-inch cake ring in the center. Place one cake circle in the center of the ring, coated side down. Pipe chocolate cream around the edge, then, starting at the center, pipe a spiral of chocolate cream to cover the cake evenly. Set the second cake circle on top, pressing down lightly to set it in place, and repeat, using enough chocolate cream to come slightly above the cake ring. Use a straight edge to smooth the cream perfectly flat.

Freeze for 1 hour, then cover with plastic wrap and freeze overnight.

Place gelatin sheets in a bowl of ice water.

In a medium saucepan, combine heavy cream, sugar, and water. Bring to a boil, reduce heat, and cook at a slow boil until reduced by about a third, 15 minutes. You should be able to spoon a bit onto a plate, run your finger through it, and not have the sauce refill the line. Remove from heat.

Squeeze excess water from gelatin sheets and whisk into the glaze.

Place parchment on a baking sheet, with a rack on top.

Remove cake from freezer and warm the cake ring with your hands until you can slide it off by pressing the cake through. Set cake on rack.

Pour glaze through a fine mesh strainer onto the cake, starting about 2 inches from the edge, until completely covered. Tap baking sheet on the counter to smooth the glaze and let set for 5 minutes.

Using a small paring knife, carefully lift a single gold leaf and place it decoratively on the cake.

Refrigerate until ready to serve.