I've made these muffins countless times - so many times that I think I'd become a bit de-sensitized by them. I remember first making them and raving to anyone who listened that these were so good and miles better than any shop-bought muffin, and they really are. But the more I made them I wished for just a little richer flavour.
I tried everything - good quality dark (plain) cocoa powder, plain choc chips - just to boost the assault on the tastebuds. And then I discovered it - and that added ingredient was..... espresso powder! Sprinkle one heaped teaspoon of espresso powder into the dry ingredients before mixing with the milk mixture and although the muffins won't taste of coffee as such, they will have a rich "roastiness" to them that will knock the chocolate flavours up a notch. Try it. I still use plain choc chips and dark cocoa powder too.
Nigella made these on TV in her Feasts programme - it's not on nigella.com but the Food Network has the recipe on its website.
Eat them, as Nigella suggests: "with the warm breath of the oven still on them". Yum.