These delicate marbled-chocolate shells make a pretty serving dish for ice cream. The scallop shells we used as molds for the chocolate measured three inches by four inches.
Prepare the scallop shells: Wrap each shell tightly in plastic wrap, making certain that the wrap is tight and smooth over the exterior (convex side) of the shell, securing excess wrap on the interior. Set aside.
Combine white chocolate and 1/2 teaspoon shortening in a medium bowl. Melt mixture gently in a double boiler or a heat-proof bowl set over a pan of simmering water. Remove from heat; set aside.
Combine the semisweet chocolate and the remaining 1/4 teaspoon shortening in a separate small bowl; melt as above.
Drizzle 2 tablespoons melted semisweet chocolate into melted white chocolate. Swirl bowl to slightly combine chocolates and to create a swirled effect. Grasping the very edges of the shell, dip the smooth exterior side of the shell into the swirled chocolate. Lift shell out of the chocolate, and tilt to evenly distribute chocolate over surface. Place shell, chocolate side up, on a rimmed baking sheet in the freezer. Repeat with remaining shells and chocolate. Freeze for at least 2 hours.
When shells are frozen, remove from the freezer. Carefully loosen the plastic wrap from edges of shell. Gently separate the shell from the chocolate. Slowly peel plastic wrap away from chocolate. Return chocolate shells to freezer, and store until ready to use, up to 1 day ahead.