First in a mug take your coffee, sugar and cocoa powder together. Here you can adjust the amount of the sugar according to your taste.
Then add the hot water little by little and mix everything properly or until dissolves.
Another way you can make the chocolate coffee mixture by using dark/milk chocolate instead of the cocoa powder. For that take 10-15 grams chocolate in a mug and add 2-3 tbsp boiling water to that. Wait for 1 minute to soften the chocolate, then mix it until dissolves. Then add the coffee and sugar. Mix again until everything is well combined.
Now in a saucepan heat up the milk on medium heat for about 3 minutes. Do not boil the milk, take it off the heat when it starts to come to a simmer. Or you can microwave the milk for 1.5 minutes on high.
Pour the hot milk into a clean heat-resistant jar and close the lid properly and be very careful here, make sure the milk isn't too hot and the bottle you're using is heat resistant. You can also use a coffee tumbler or flask instead of a glass jar.
Now shake the bottle for 15-20 seconds until the milk becomes frothy.
Then slowly pour the frothy milk into the chocolate coffee mixture and your cafe mocha is ready to drink.
You can decorate your coffee with some cocoa powder or chocolate syrup on top.