01 - Gently melt dark chocolate in a heatproof bowl set over simmering water or in short bursts in the microwave until smooth.
02 - Apply melted chocolate to silicone dome molds (3-inch diameter) using a pastry brush or spoon, ensuring an even layer. Chill molds for 10 minutes. Apply a second layer for reinforcement, then chill until fully set, about 30 minutes.
03 - Soften gelatin leaves in cold water for 5 minutes or, if using powdered gelatin, bloom in 2 tablespoons cold water. In a mixing bowl, whisk mascarpone, powdered sugar, and vanilla until combined. Whip cream to soft peaks and delicately fold into mascarpone mixture.
04 - Gently dissolve gelatin by heating, then incorporate into the mascarpone mixture. Pipe or spoon mousse into chocolate shells, filling nearly to the top, and smooth the surface. Refrigerate at least 3 hours, until mousse is set.
05 - Carefully unmold the domes and place on a wire rack set over a tray.
06 - Soften gelatin in 3 tablespoons cold water. In a saucepan, combine water, sugar, cream, and sifted cocoa powder. Bring to a simmer while whisking until smooth. Remove from heat; cool slightly to 122°F, then stir in softened gelatin until fully dissolved. Allow glaze to cool to 90°F for optimal pouring consistency.
07 - Pour mirror glaze evenly over each mousse dome to completely coat, allowing excess to drip off. Carefully transfer domes to serving plates.
08 - Finish domes with gold leaf, chocolate curls, or fresh berries as desired. Chill for 30 minutes to set the glaze before serving.