This is the best chocolate muffin recipe I've come across, it's from http://www.thepinkwhisk.co.uk/2011/03/chocolate-muffins-and-the-perfect-chocolate-cake-recipe.html.
As recommended in the recipe, I use a Masterclass Flavour Injector which I bought from Lakeland to get the chocolate sauce inside, I've tried other methods but they're quite fiddly so would recommend buying one of these if you intend on making these regularly! See http://www.lakeland.co.uk/13463/Flavour-Injector;jsessionid=F16E0D6AADF0220BE41A131D8E52490A.app1
(The tulip cases were from Sainsbury's, £2.99 for 12. Alternatively, you can use standard muffin cases which will probably fill about 16+)