Melt 6 ounces of the dark chocolate in a double boiler until it reaches 115°F (46°C). Remove from heat and stir in the remaining 2 ounces to temper, stirring until smooth and cooled to 90°F (32°C).

Coat the interiors of 6 silicone muffin molds with a thick, even layer of the tempered chocolate using a pastry brush or spoon.

Place the molds in the refrigerator for 20 minutes or until the chocolate is completely set and firm.

While shells set, combine heavy cream, confectioners' sugar, and vanilla extract in a chilled mixing bowl.

Whisk the cream mixture on medium-high speed until stiff peaks form, taking care not to over-beat.

Carefully unmold the chocolate shells by gently peeling the silicone away from the edges.

Pipe or spoon the whipped cream into each chocolate shell until three-quarters full.

Arrange fresh berries on top of the cream and garnish each cup with a mint sprig.

Keep refrigerated until ready to serve.