I have lots of great from-scratch chocolate cake recipes, one in particular that's very quick and easy, but they all call for two 9-inch layer rounds. You can substitute two 8x8 inch squares for the rounds, but you only have one, so you would need to divide the batter in half, and do one layer at a time, which may compromise the rise on the second layer. Otherwise, do half the recipe altogether as one layer.
Another idea is to do a flourless or almost flourless chocolate cake/torte. They're usually one-layer cakes, and you can use the 8x8 inch in place of a 9 inch round. I have several good recipes for those also.
If you need cupcake recipes, there's a recent thread here on chocolate cupcakes. You can also do lava cakes/molten chocolate cakes.
Let me know if you want links to a few of the flourless or the regular cake recipes.
"Strength is the capacity to break a chocolate bar into four pieces with your bare hands - and then eat just one of the pieces."