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.