King Arthur Flour Chocolate Breakfast Muffins are intensely chocolate, very moist (if you weigh your ingredients properly) and have nice high tops.
The recipe calls for 1/2 cup of butter or 1/3 cup of vegetable oil. For the first batch, I used half of each because I couldn't decide which. I used 4 tablespoons of butter and a little less than 3 tablespoons of oil. For my second batch I just used 1/3 cup of oil. I prefer the oil version better. I love butter, in this case there's so much chocolate flavor you don't really miss the butter. Also, oil seems to give the muffins a little extra moisture.