You can easily melt the chocolate, butter, and oil in the microwave. Just make sure you do it in intervals, taking the bowl out once in a while to give it all a good mix. If you go the double boiler route, make sure the water doesn’t touch the bottom of the bowl. You want the heat, but you don’t want to risk the water getting into the bowl because it’ll cause the chocolate to seize on you rather than melt.
Make sure the melted chocolate is no longer warm to the touch–you don’t want it to scramble the eggs when you mix the two together.