In a small saucepan, melt butter and chocolate on low heat so it doesn't burn.

In another medium sized saucepan, whisk the other ingredients together over medium heat — keep stirring as it comes to a low boil and it'll start to thicken after it boils for a minute or two.

Remove from heat and whisk in the melted chocolate.

Let cool a little bit before putting it onto your ice cream, it'll be a little thicker that way, plus it won't melt it all.  :)