First, don't worry about your recipe being too similar to anyone else's. There is a finite amount of ingredients, and the number of combinations that will make a good example of a given style is even smaller. There is no ethical or moral requirement that your recipes be exclusively yours! Now, if you do
use someone else's recipe, it's always nice to give credit where it's due.
Looks like it'd be a good beer as written. I'll offer you a few possible tweaks, though:
Dark malt extract is made with Pale, Crystal and Black malts. If you use Light or Pale malt extract, you have much greater control over the color and flavor of the beer, because you use specialty grains to impact the color and flavor(s).
Think of Pale Malt and Pale/Light extract as a chicken breast. Right out of the package, a chicken breast is pretty bland. But when you start adding spices - cajun spices, garam masala, jerk seasoning - you end up with a completely different flavor, the levels of which you control. Now, think of Dark extract as a chicken breast you buy pre-packaged in "Italian Marinade". Do you have control over the flavor of that chicken breast? Nope.
I'd advise replacing your corn syrup with the same amount of Pale/Light malt extract. Pure corn sugar will thin the beer's body; sometimes that's desireable, but in this case I think it's excessive. Tell you what - here's something that's worked for me. Replace one pound of your sugar with a pound of Dry Malt Extract, and the other pound with a 454-gram tin of Lyle's Black Treacle or 16 oz bottle of molasses. That'd be an interesting flavor!
Now, I also think you have too much Black Patent malt. That stuff is pretty potent, and a half-pound can easily overpower a beer with burnt, ashy, phenolic flavors. I recommend using a quarter pound in Robust Porter (half your listed amount of Chocolate).