Line a deep dish or a baking tray with a parchment paper or grease with a few drops of ghee and keep aside. A size of 8x4" will do, but if you have a bigger thaali / dish, make sure you don't spread over the entire surface or the burfis will be very thin and fragile.
In a heavy bottomed pan, place the milk, sugar, ghee and rava. Bring to boil on a high flame with constant monitoring and stirring.
Once it comes to a boil, reduce flame and allow to simmer with reasonably constant stirring to make sure the milk does not burn or catch at the bottom of the pan, or doesn't boil over.
After stirring and simmering for around 20 minutes, the milk will turn much thicker, roughly the consistency of dosa / pancake batter. Whisk the cocoa powder in warm milk and pour into the thickening milk with constant stirring.
In another 5-8 minutes (continue stirring), the mixture will turn much thicker leaving the sides of the pan. At this point switch off flame, because it will continue to thicken in the residual heat.
Pour / transfer the mixture over the greased parchment, taking care to spread over a smaller area if you want thicker burfis.
Let this cool for 15 minutes after which you can cut into squares / rectangular bars with a sharp knife or a pizza cutter.
Store in airtight container and consume in 3-4 days. Keeping them in the fridge makes them too hard and I personally don't like them cold.