01 - Melt dark chocolate in a heatproof bowl over gently simmering water or in the microwave in 20-second bursts. Using a pastry brush or spoon, coat silicone dome molds (2.75–3 inch diameter) with a generous layer of melted chocolate. Chill for 10 minutes, then repeat to thicken. Chill thoroughly for about 30 minutes until firm.
02 - Soften gelatin leaves in cold water for 5 minutes or dissolve powdered gelatin in 2 tablespoons cold water. In a mixing bowl, whisk mascarpone, powdered sugar, and vanilla until smooth. Whip heavy cream to soft peaks and gently fold into the mascarpone mixture. Warm gelatin until dissolved and incorporate into mousse base. Spoon or pipe the mousse into the set chocolate shells, filling almost to the rim and smoothing the tops. Refrigerate for at least 3 hours to set completely.
03 - Carefully release the domes from silicone molds and arrange them on a wire rack set over a tray to catch excess glaze.
04 - Soften gelatin in 3 tablespoons cold water. In a saucepan, combine water, sugar, heavy cream, and sifted cocoa powder; bring to a gentle simmer, whisking until glossy and smooth. Remove from heat, cool slightly to 122°F (50°C), then add gelatin, mixing until dissolved. Let glaze reduce to 90°F (32°C) for optimal pouring consistency.
05 - Pour the mirror glaze evenly over each dome, ensuring complete coverage. Move domes onto dessert plates. Finish with edible gold leaf, chocolate curls, or fresh berries if desired. Chill domes at least 30 minutes to set the glaze before serving.