01 - Melt dark chocolate in a heatproof bowl over simmering water or using microwave intervals. With a pastry brush or spoon, coat silicone dome molds (3-inch diameter) with an even layer of melted chocolate. Refrigerate molds for 10 minutes, then apply a second coat for thickness. Chill until the shells are firm, about 30 minutes.
02 - For gelatin sheets, soak in cold water for 5 minutes; for powdered gelatin, sprinkle over 2 tablespoons cold water to bloom. In a mixing bowl, combine mascarpone, powdered sugar, and vanilla, whisking until smooth. In a separate bowl, whip heavy cream to soft peaks and fold gently into the mascarpone mixture. Gently warm bloomed gelatin until dissolved, then fold into the mousse mixture. Pipe or spoon mousse into the chocolate shells, filling almost to the top and smoothing the surface. Refrigerate for at least 3 hours, until fully set.
03 - Carefully release the filled domes from the molds. Place them on a wire rack positioned over a tray to catch excess glaze.
04 - Soften gelatin with 3 tablespoons cold water. In a saucepan, blend water, sugar, heavy cream, and sifted cocoa powder. Bring to a gentle simmer, whisking until smooth. Remove from heat and allow to cool slightly, around 122°F. Stir in the softened gelatin until fully dissolved. Let the glaze cool further to 90°F to achieve an ideal pouring consistency.
05 - Pour the mirror glaze smoothly over each dome, ensuring even coverage and allowing any excess to drip away. Transfer domes to serving plates. Garnish with gold leaf, chocolate curls, or fresh berries as desired. Refrigerate for at least 30 minutes to set the glaze before serving.