01 - Melt dark chocolate in a heatproof bowl set over simmering water or in short microwave bursts until smooth. Using a pastry brush or spoon, coat silicone dome molds (3-inch diameter) evenly with melted chocolate. Chill for 10 minutes to set, then apply a second coat for sturdier shells. 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, whisk together mascarpone, powdered sugar, and vanilla until combined. Whip heavy cream to soft peaks and gently fold into the mascarpone mixture. Warm dissolved gelatin until just melted, then blend into the mousse. Spoon or pipe mousse into the prepared chocolate shells, filling almost to the brim, and smooth the surface. Refrigerate domes for at least 3 hours until set.
03 - Carefully remove chilled domes from silicone molds and set on a wire rack placed over a tray.
04 - Bloom gelatin in 3 tablespoons cold water. In a saucepan, combine water, sugar, heavy cream, and sifted cocoa. Whisk continuously and bring to a gentle simmer. Remove from heat and let the mixture cool slightly (to about 122°F), then stir in bloomed gelatin until fully dissolved. Allow glaze to cool to approximately 90°F for proper pouring consistency.
05 - Pour mirror glaze evenly over each dome, allowing excess to drip off for even coating. Carefully transfer domes to dessert plates. Optionally decorate tops with gold leaf, chocolate curls, or fresh berries. Chill domes for at least 30 minutes to allow glaze to fully set before serving.