You can have it as such without inverting the cake.
You can use any cupcake moulds or some type of oven proof moulds you have.
Do not over cook the batter or else it will become a cake.
The lava should ooze out when cutting the cake for that we must cook it for lesser amount of time.
Oven temperature differs from oven to oven so adjust accordingly.
Instead of butter you can use any flavourless oil.
It is best to make smaller pieces of chocolate to place in the centre of the cake batter.