In a microwave-safe bowl, melt the chocolate chips in 30-second intervals until smooth.
Place one Tablesppon of the melted chocolate in the center of each mold. Use a pastry brush or spoon to swirl the chocolate around the mold to cover all sides.
Transfer the mold to the freezer for 3 minutes.
Remove the mold from the freezer and repeat the same process. Continue to fill the molds one tablespoon at a time, freezing for 3 minutes between each layer until all the chocolate has been used. Once all the chocolate has been used, freeze for 15 minutes.
Carefully remove the spheres from the mold cavities and set aside.
In a small bowl, combine the sugar, cacao powder, and sea salt. Divide the mixture and the marshmallows between 3 of the sphere halves.
Warm a plate in the microwave for 30-45 seconds until hot. Place the unfilled sphere halves on the warm plate, allowing the edges to melt for about 5 seconds.
Carefully sandwich the filled and unfilled spheres together. If desired, add a bit of melted chocolate to a piping bag and go around the center of each bomb to create a tighter seal.
Drizzle with additional chocolate if desired and top with optional toppings.