01 - Gently melt dark chocolate in a heatproof bowl over simmering water or in the microwave using 10-second bursts. Brush or spoon a generous layer into silicone dome molds (3-inch diameter) ensuring full coverage. Chill for 10 minutes. Apply a second coat and refrigerate until completely firm, about 30 minutes.
02 - Soak gelatin sheets in cold water for 5 minutes or bloom powdered gelatin in 2 tablespoons cold water. In a large mixing bowl, whisk mascarpone, powdered sugar, and vanilla until smooth. Whip heavy cream to soft peaks and carefully fold into mascarpone base. Gently warm the gelatin until fully dissolved, incorporate into the mousse, and blend thoroughly.
03 - Transfer mousse into the chocolate shells using a spoon or piping bag, filling nearly to the top. Level the surface with an offset spatula. Refrigerate domes for a minimum of 3 hours until completely set.
04 - Unmold each dome by pressing gently from the base and place onto a wire rack set over a tray to catch any glaze drips.
05 - Soften gelatin in 3 tablespoons cold water. In a saucepan, combine water, sugar, heavy cream, and sifted cocoa powder. Bring the mixture to a gentle simmer, whisking until glossy and lump-free. Remove from heat and cool to 122°F. Dissolve softened gelatin into the mixture. Let the glaze further cool to 90°F for flawless pouring.
06 - Pour mirror glaze over each chilled dome, allowing excess to drip off. Shift domes onto dessert plates. Embellish with edible gold leaf, chocolate curls, or fresh berries as desired. Return domes to the refrigerator for 30 minutes to set the glaze before serving.