cant see very well what’s happening in the picture with the full block, but to chisel down to fill a chip like you have there, you shouldn’t go for the diagonal shape (which is flatten block with precise chisel removing diagonal corners),
if you are trying to fill two corner gaps like that with one block, you won’t be able too, because (as @Nightstar explained) you cannot remove the middle of the diagonal shape to leave opposite corners floating
chiseling one block can only fill one corner chip (gap) like that and to do that you follow these steps (you probably know that):
in that case you would have to go for double the size so you need 2 blocks to span the arms of the pattern - that way you will have chips filled in both arms
The shape (block) you want …
… is one of many shapes we are missing,
and we all waiting for at the moment.
Who knows, maybe we get some of them next year
You may be able to get what you want if you make that same shape with precise bevel chiseling… it’s basically a half block offset of the bevel chisel. May still not work but it’s a thought.
Oh wait I may be thinking precise slope chiseling, I’m a little rusty. There is an option where you precise bevel chisel all but opposite diagonal corners to get one block split in two but it doesn’t work here.
I was trying to answer this, since I’m pretty sure I know how to do it, but I can’t figure out how to articulate it better than “well, you just stand at the top and use the precise bevel chisel to make it diagonal”, which I’m not sure is actually useful.
If I ever get done with my other projects, I want to make a chisel school on someone’s creative world.
I think it’s 4 x 4(2x2 in the centre with deco gleam) x 6 blocks high.
Bevel the side down to a half block. Then precise bevel chisel out the centre and top and bottom if that makes sense.