Don't worry you cannot change it now, instead focus to organize and have a lot of carbs plus some greens.
A vegan muffin? 18 grams of fat? ...and you're worrying about gaining weight FROM THAT?
I don't want to sound harsh, but I think you have a bigger problem than having eaten that vegan muffin. I mean, this can't be real!
I feel your pain, Hopefully you are recovered. Focus on the great things you've done for your body and forget about the muffin slip-up. You can accomplish your goals!