To make it easy and cheap, I have used homemade chocolate syrup. For better taste you can use melted chocolate.
I have used Sweetened cocoa powder to make this. You can use unsweetened cocoa powder also, in that case you may get bitter taste.
Or You can watch 3 ingredients 15 minutes choco lava cake https://youtu.be/cbcfMUcAZNs
You don’t need any cocoa powder to make this.