Flat or 3D? For flat, it's cutting a round cake to shape (use the cutout for the face to become the face mask). Use a soft frosting for the cut edges or you will tear the cake up. Or find a helmet cake pan : )
For 3D, you could use a bunt cake pan, but you would have to fill the center, so you would need to make layers. Either have graduated cake pans or make big sheet cakes and cut them to size.
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I like the idea of using a Bundt pan - then you could fill the center with some sort of tasty filling, then top with concentric circles from a second cake to make the top of the helmet round. An arc cut from the edge of a round cake could also be attached to the front for the face guard.
I'm sorry, but filling a helmet cake up with tasty filling that oozes out when you cut it open? Sometimes it could be appropriate, but I don't think it's for Halloween : )