So, what setting are you using to melt butter?

When I want to melt (or soften) margarine or butter, I set my microwave on a "3", or "4" - at the highest.  Set it too high, and it wants to spatter.

The other thing I have done with some foods is to take a water glass, fill it about 1/4- 1/3 full, and set it in the microwave while cooking some foods. I than can set the microwave a bit higher.  I think the water helps to absorb some of the excess energy, so foods don't spatter so much.  You will have to experiment to determine actual settings and times.