This is a recipe for yummy, moist and bouncy muffins where you can add anything to it like coca powder, dried fruit etc. I've made muffins with this recipe many times now and they come out beautifully, especially in silicone muffin cases. I recommend using dark chocolate, it makes the muffins less sweet.
When cooling the melted butter down to room temperature, a quick way to cool it without effecting it is by putting some cold water in your sink and place the bowl into it.