In a mixing bowl, whisk together the glutinous rice flour, sugar, cacao powder, water, coconut milk, and vanilla. The mixture will be liquid but will thicken once cooked. Transfer to a glass or ceramic bowl/dish.
Steam for about 20 minutes. To steam, I recommend you fill a large pot with about 2 cups of water, then place a metal basket inside the pot and place your bowl on it. Cover with a lid wrapped with a kitchen towel (to prevent water from dropping into the mochi dough).
After 20 minutes, you should have a very sticky dough. Let it cool a few minutes, then cover with plastic film to touch and refrigerate for about 2 hours, or until cold. If the dough is warm it will not be easy to work with and could melt the chocolate ganache.
To assemble: place the balls of ganache in the freezer for about 10 minutes. Dust your working surface with cornstarch. Scoop out about 1,5 tbsp of mochi dough and place on the cornstarch surface. Slightly flatten into a round and dust with more cornstarch so it's not too sticky.
Place a ball of ganache in the center of the mochi round. Wrap the dough around the ganache and pinch the edges to seal. You can use plastic film to handle the dough if it's too sticky. Dust off excess cornstarch, and place your mochi on a plate line with parchment paper. Repeat with the remaining dough and ganache. Dust with cacao powder, cover with plastic film, or place in a airtight container and refrigerator for at least 2 hours before enjoying!
You can enjoy mochi straight from the refrigerator or leave it out for 5-10 minutes before eating. Mochi is best eaten the same day but will keep for up to 2 days in the refrigerator.