This site is very helpful in determining how toxic the quantity is.
Chocolate and Dog Poisoning
I would be extremely concerned if the ate the cocoa - but once you mix in the flour, eggs, butter, etc., you are probably looking at more of an upset tummy. Do a little math and compare the quantity he ate to that site and see what it says.
I hope that helps! There is also a dog poison control hotline if you can't reach a 24 hour vet. I hope he is OK!