For the hazelnut filling: Heat the chocolate chips and heavy cream together in a small microwaveable bowl for 30 seconds, then stir until smooth and homogenous. Stir in the hazelnuts and freeze until firm, 15 to 20 minutes.
For the pistachio marzipan: Blend the pistachios in a food processor until they're finely ground (but don't overblend or you'll end up with pistachio butter). Add the powdered sugar, almond extract and salt and pulse to combine. With the motor running, drizzle in the corn syrup and blend to form a dough. (It may still look crumbly in the food processor, but if you squeeze it in your hand it should stick together.) Pile it into a ball and wrap it in plastic. Refrigerate at least 1 hour, or up to overnight.
For the almond marzipan: Combine the almond meal, powdered sugar, almond extract and salt in a food processor and pulse to combine. With the motor running, drizzle in the corn syrup and blend to form a dough. (It may still look crumbly in the food processor, but if you squeeze it in your hand it should stick together.) Pile it into a ball and wrap it in plastic. Refrigerate at least 1 hour, or up to overnight.
To assemble, divide both the almond marzipan and the pistachio marzipan into 12 equal portions. Keep the marzipan covered with plastic wrap when you're not working with it. Take one of the pistachio marzipan portions and smash it with your palm to make a flat round. Fold it over a rounded 1/4 teaspoon hazelnut filling and roll it into a ball. Smash one of the almond marzipan portions with your palm and fold that over the pistachio ball. Roll it into a ball and place it on a parchment-lined baking sheet. Repeat with the remaining marzipan and filling and freeze the balls for 15 minutes.
For the coating: Melt the dark chocolate either in a double boiler or microwave, heating in 30-second increments and stirring after each, until it is smooth and melted. Continue stirring until it is no longer hot to the touch.
One by one, dip the marzipan balls in the chocolate to fully coat. Allow the excess chocolate to drip off, and then place back on the parchment. Sprinkle with some sanding sugar. Harden them in the refrigerator or at room temperature for 1 hour or up to overnight.