01 - Melt the dark chocolate in a heatproof bowl over gently simmering water or in the microwave using short intervals until smooth. With a pastry brush or spoon, coat the interiors of silicone dome molds (approximately 3-inch diameter) with an even layer of melted chocolate. Refrigerate for 10 minutes, then repeat with a second layer for thickness and chill 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 for 5 minutes. In a mixing bowl, combine mascarpone, powdered sugar, and vanilla using an electric mixer or whisk until smooth. Whip heavy cream to soft peaks separately, then gently fold into the mascarpone mixture. Dissolve softened gelatin by briefly heating, then incorporate into the mousse. Spoon or pipe the mousse into the chilled chocolate shells, nearly filling each one. Smooth the tops and refrigerate for at least 3 hours until fully set.
03 - Carefully unmold each chocolate-filled dome and transfer to a wire rack set over a tray to prepare for glazing.
04 - Soften gelatin in 3 tablespoons cold water. In a small saucepan over medium heat, combine water, sugar, heavy cream, and sifted cocoa powder. Whisk constantly and bring just to a simmer until the mixture is homogeneous. Remove from heat and let cool slightly to about 122°F. Stir in the softened gelatin until fully dissolved. Allow the glaze to cool further to about 90°F, reaching optimal pouring consistency.
05 - Pour chocolate mirror glaze evenly over each dome, ensuring the shells are fully coated. Allow excess glaze to drip off. Transfer domes carefully onto dessert plates. Garnish with gold leaf, chocolate curls, or fresh berries, as desired. Refrigerate for at least 30 minutes for the glaze to set before serving.