How to achieve a good steak crust with a blowtorch?
It does take a couple of minutes to get a good crust with a torch, but judging by that photo something went wrong as there’s little crust on that picture.

As aris mentioned, you need to ensure that there is no moisture on the steak.
You mentioned adding butter on top and letting it melt at room temperature. This could act as a barrier for the heat but no kitchen test I know of tackled this specifically. Next time try without as I’ve always gotten and seen good results without butter on it.

Lastly, keep the torch at a moderate distance and linger slightly on a spot until you see some sizzling. Make small circles, but don’t move the torch all over rapidly. You don’t need to distribute the heat evenly while trying to get a crust.