Muffin Mysteries

About nine years ago, I got an email from an old school friend. It only said one thing, which was something like:
"How do you make muffins do the spilling-over-the-top thing?"

I still don't know. Any cupcakes I make are flat at the top. I did once get the desired effect when I used bicarbonate of soda instead of baking powder. They looked fantastic but tasted disgusting. I've just looked on the internet and one suggestion is to use more baking powder. I might try that next time.

My next question is: how do you keep chocolate chips on the top from burning? And how do you help chocolate chunks which are slightly cuboid keep their shape?

Today I made some chocolate fairy cakes which had chocolate chips and a bit of a gooey toffee centre. I was inspired by a muffin I bought in a theatre cafe on Thursday evening. It was chocolate batter with chocolate chips and had little squidges of toffee inside. It was lovely.

I did buy milk chocolate to break into bits and pop into the cake mix, but then I was struck with an idea which would use up some leftovers, thus creating more space in the fridge.
I had some chocolate sauce in the fridge which had been made for the ice-cream cake I made recently. The cake had long since gone, and I was left with the chocolate sauce made of chocolate and double cream which, as you might expect, had the hardness of truffles. I chopped it up into little chunks and shoved bits into the cake mix. I half-filled the cake cases, then dribbled in some toffee sauce which I had also made to accompany the long-gone ice-cream cake. I made sure the toffee got covered with cake mix.

I got a pretty good result. The toffee sauce didn't go hard (good). The exposed bits of chocolate chunk did not burn (good) but spread a little and did not keep their shape (disappointing).

They taste very nice, by the way, but I would still like your hints for spilling-over-the-top chunky choc chip muffin success.