In a bowl. Add cocoa powder. Corn flour and 6 tbsp milk and make slurry. Make sure there are no lumps.

Heat 500 ml milk till simmering stage. Then slowly add slurry. And mix properly

Add sugar and mix well and cook for 2-3 min.

Then add dark chocolate. And mix well and cook for 2-3 min on low flame.

Once mixture start boiling. Switch off the flame and keep a side till comes to room temperature.

Refrigerate for 2-3 hours. And then enjoy Your cold cocoa.