Step 1

Place the chocolate chips in a small bowl. Microwave the cream in 20-second bursts until hot to the touch, then pour it over the chocolate chips. Let sit for 1 minute, then whisk together until the chocolate is melted and the mixture is smooth and glossy. Refrigerate until firm enough to scoop but not hard and brittle. Meanwhile, prepare the sesame seed filling.

Step 2

If using whole sesame seeds, use a mortar and pestle or spice grinder to grind them to a powder; you should have about 2 tablespoons. Heat the coconut oil in a small saucepan over low heat. Remove from the heat, add the granulated sugar and ground sesame seeds or sesame seed powder, and stir until combined. Transfer to a small bowl and refrigerate until firm enough to scoop. Meanwhile, make the dough.

Step 3

Place the sweet and rice flours in a large bowl and whisk to combine. Add 1 cup of water and stir to combine, then knead with your hands until the dough is smooth and no longer sticky. Form the dough into a ball, then divide into 2 pieces. On a rice flour-dusted surface, roll one piece into a 12-inch log. Cut crosswise into 12 pieces. Repeat with the remaining piece of dough. Transfer the pieces to a rimmed baking sheet and cover with plastic wrap until the fillings are ready.

Step 4

When the chocolate filling is set but scoopable, scoop out 12 heaping 1/2 teaspoons of filling, roll each into a ball, and place on a large plate. (Save the remaining chocolate filling for another use. It makes a great hot chocolate!) Repeat with the black sesame filling, placing the balls on the same plate.

Step 5

To form the rice balls, take a piece of dough and press it into a 1/2-inch thick disc. Place a ball of chocolate or sesame seed filling in the center of the disc, then wrap the dough around the filling, pinching the dough as needed to completely enclose the filling with dough. Roll into a smooth ball and place back on the baking sheet. Repeat with the remaining dough pieces and filling.

Step 6

When you are ready to cook the rice balls, fill a large pot with water and bring it to a simmer (a furious boil will break open the rice balls) over medium heat. (If you want to keep the black sesame and chocolate rice balls separate, cook them in two separate pots of water.) Gently place the rice balls in the water, and simmer until they expand slightly and are almost translucent, about 15 minutes. Stir occasionally to keep the rice balls from sticking to the bottom of the pot.

Step 7

Meanwhile, bring 6 cups of water to a simmer in a medium pot over medium heat. Add the rock sugar and cook until the sugar is melted and the syrup is simmering; set aside. You want to serve this dish hot, so bring the syrup back to a simmer when you are ready to serve.

Step 8

To serve, place rice balls (2 chocolate and 2 sesame, or more if desired) in each bowl and top with syrup.