You dissolve butter and chocolate over steam, and then let cool down on the table

Eggs (4 whole eggs and 4 yolks) and sugar go into a bowl where you mix them with mixer

After eggs and sugar are smooth, you add flour and the butter-chocolote mix that had cooled down

Then you fill the muffin molds with the the mix and get it into the owen at 180° for 7-9 minutes, you can leave it for longer of it's not baked enough yet.
The magic in making lava muffins is in getting it out of the owen at the right time.
When it looks similar to the image below, quickly get it out of the owen, it's ready.
If you want solid muffins, you can leave it for a while longer.