I think you have actually cooked the chocolate instead of melting it. The wooden spoon also could be the culprit.

I usually use a glass bowl and metal spoon. Make sure you don't have too much water in the saucepan so that it doesn't touch the bottom of the bowl and produce too much heat. The water just needs to create steam to melt the chocolate. Also, make sure you keep stiring until the chocolate has melted.

Good luck!