99% of the time, the head gasket itself isn’t to blame when it blows. Its usually loose head bolts/studs, cracked bore or head (due to overheating or sheer inability to handle massive boost), incorrectly machined mating surfaces, etc.

This is also backed up by the fact that the stock STi gaskets, let alone the Cosworth ones, are more than capable of handling anything you can throw at them (in combination with some decent head studs), as witnessed by numerous builds over the years...

I’d be going with the Cosworth gaskets again, after you’ve determined the underlying problem.