01 - Melt dark chocolate in a heatproof bowl set over simmering water or in the microwave using short intervals. With a pastry brush or spoon, generously coat silicone dome molds (2.75–3.15 in diameter) with chocolate. Chill for 10 minutes to set, then repeat to build a thicker shell. Refrigerate until fully set, approximately 30 minutes.
02 - Soften gelatin sheets 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 until smooth. Separately whip cream to soft peaks, then fold gently into mascarpone mixture. Warm gelatin until fully dissolved and fold into the mousse base. Gently spoon or pipe the mousse into the chocolate shells until nearly full and smooth the tops. Refrigerate at least 3 hours to set.
03 - Carefully unmold the domes from the silicone molds and place each on a wire rack set over a tray.
04 - Soften gelatin in 3 tablespoons cold water. In a saucepan, gently combine water, sugar, heavy cream, and sifted cocoa powder. Bring to a simmer while whisking until fully blended. Remove from heat and allow to cool briefly (to approximately 122°F), then add the softened gelatin and stir until dissolved. Allow to cool to about 90°F for ideal pouring consistency.
05 - Evenly pour the mirror glaze over the domes so it coats and drips down the sides. Transfer to serving plates. Garnish each dome with gold leaf, chocolate curls, or fresh berries if desired. Chill domes at least 30 minutes so the glaze can set before serving.