Wondra is a product made by combining very low (finely ground) protein flours.
There is a sub online with 1 cup AP and 1/2 tsp cornstarch but I don't thing that small amt of starch will transform AP flour into anything earth shattering ....
MIGHT help with the lumpy gravy as corn starch has anti clumping properties but again at such a small ratio I would have to see it to recommend it.
Is this the recipe on the label http://www.recipelink.com/msgbrd/board_0/1997/OCT/10427.html
While the Gold Medal site had quite a few chocolate cake recipes the one you cited was not there.
Note that like brownies this "cake" recipe uses eggs as a leavening so I find it hard to believe (even in the recommended 8x8 pan) that it will rise very much (cake like) and if it does the flavor of egg will overwhelm the product.
The cocoa powder is being used not only for flavor but also a bit of extra structure so that may help a bit.
VERY fudgy outcome and I expect more brownie than cake like.
If you do end up making it and it works for your needs try substituting a 1/4 cup of brown sugar for the granulated as brown sugar and chocolate paired will transform an everyday brownie into something special.