In a small bowl, combine custard powder, cocoa powder and ¼ cup milk. Mix well till no lumps remain. Keep aside.
Now heat the remaining ¾ cup milk on medium flame. Once it becomes hot, add sugar. Mix well till sugar dissolves.
Once sugar is dissolved, add custard+milk mixture to it and keep stirring continuously with a whisk. If you feel that you are not able to manage with medium flame, reduce the flame to low.
Keep whisking continuously till you notice that the mixture has started thickening. Once you notice that mixture has thickened, remove from flame.
Let the mixture cool down a bit for about 10 minutes.
Whisk again and now pour the mixture in serving bowl/glasses and fill till half. Keep space for the froth. Refrigerate for atleast 1 hour.
Assembling pudding
For the froth (Do this process just before serving)
In a bowl combine sugar, coffee and hot water. Whisk till it becomes frothy. If using electric whisk it will take about 2-3 minutes. If you are doing it with a whisk manually, it will take about 10 minutes.
Put froth over the pudding and serve immediately. Yummy dalgona mocha pudding is ready.