01 - Melt dark chocolate in a heatproof bowl over a pan of simmering water or in short bursts in the microwave until smooth. Coat silicone dome molds (3–3.25 inch diameter) thoroughly with melted chocolate using a pastry brush or spoon. Chill molds for 10 minutes to set the layer, then repeat for a thicker shell. Return to the refrigerator until completely set, about 30 minutes.
02 - Soften gelatin leaves in cold water for 5 minutes or, if using powdered gelatin, sprinkle over 2 tablespoons cold water and let bloom. Whisk together mascarpone, powdered sugar, and vanilla in a mixing bowl until smooth. Whip heavy cream to soft peaks. Gently fold whipped cream into mascarpone mixture. Gently heat gelatin until fully dissolved, then fold into mousse mixture.
03 - Spoon or pipe the mascarpone mousse into the prepared chocolate shells, filling each nearly to the top. Smooth the surface, then refrigerate domes for at least 3 hours or until the mousse is fully set.
04 - Carefully unmold the domes and set them on a wire rack placed over a tray to catch excess glaze.
05 - Soften gelatin in 3 tablespoons cold water. Combine 0.42 cup water, sugar, heavy cream, and sifted cocoa powder in a saucepan. Bring to a simmer, whisking until smooth. Remove from heat and cool to 122°F. Stir in softened gelatin until fully dissolved. Allow glaze to cool further to 90°F for perfect pouring consistency.
06 - Pour mirror glaze over each dome on the wire rack, ensuring even coating. Let excess drip off. Use two spatulas to carefully move domes to dessert plates. Decorate with gold leaf, chocolate curls, or fresh berries as desired. Chill domes at least 30 minutes to set the glaze before serving.