a 10-inch-tall chocolate plastic bunny mold (with 2 halves)
Add half of the chocolate to a medium microwave-safe bowl and microwave in 30-second intervals, whisking in between each, until the chocolate is melted, about 2 minutes. Add the remaining chocolate and continue to whisk until smooth; this will result in a tempered chocolate that will appear glossy when set. Transfer 1/4 cup of the tempered chocolate to a small bowl; set aside.
Divide the remaining chocolate between the 2 halves of a 10-inch-tall plastic bunny mold. Tilt the mold halves to swirl the chocolate around until each is coated entirely. Refrigerate until set, about 15 minutes.
Add the marshmallow creme, chocolate-hazelnut spread and caramel syrup to a medium bowl and fold with a rubber spatula to combine. Coat 2 small rubber spatulas with nonstick spray and use them to divide the mixture between the mold halves. Refrigerate until firm, about 15 minutes.
Reheat the reserved 1/4 cup of chocolate in 15-second intervals, whisking in between each, until the chocolate is melted, about 45 seconds. Transfer to a piping bag; snip the tip off to make a 1/8- to 1/4-inch hole. Pipe the melted chocolate around the edge of one of the mold halves. Sandwich both halves together and refrigerate for 10 minutes.
Carefully unmold and trim away any excess chocolate around the edges if needed.