As I did not have a double boiler, I simply place a stainless steel bowl, in a pot (half filled with water) boiling on the stove to melt my chocolate and butter together.
I bought my remekin from Daiso and it has a diameter of 12cm, thus I only need two of it to contain all the quantity I made above.
By the way, this is the second time I made this chocolate lava cake! The first time, it turn out to be a chocolate cake without any lava oozing out (picture below), haha! ;p