01 - Melt dark chocolate in a heatproof bowl over simmering water or microwave in short bursts. Using a pastry brush or spoon, coat silicone dome molds (3–3.25 inch diameter) with a generous chocolate layer. Chill for 10 minutes, then repeat for a thicker shell. Refrigerate until fully set, about 30 minutes.
02 - Soften gelatin leaves in cold water for 5 minutes, or bloom powdered gelatin in 2 tablespoons cold water. In a mixing bowl, blend mascarpone, powdered sugar, and vanilla. Whip heavy cream to soft peaks, then gently fold into mascarpone mixture. Melt gelatin until dissolved, incorporate into mousse. Fill chocolate shells, smoothing the tops. Refrigerate at least 3 hours until set.
03 - Carefully remove domes from molds, placing each on a wire rack set over a tray.
04 - Soften gelatin in 3 tablespoons cold water. In a saucepan, combine water, sugar, heavy cream, and sifted cocoa. Bring to a simmer, whisking until smooth. Remove from heat and cool slightly (to around 122°F), then stir in softened gelatin until dissolved. Let glaze cool further to about 90°F for ideal pouring consistency.
05 - Pour mirror glaze evenly over each dome, allowing excess to drip off. Transfer domes to dessert plates. Garnish with gold leaf, chocolate curls, or berries if desired. Chill domes at least 30 minutes for glaze to set before serving.