If you do not have any mint extract and want to make something easy and delicious without turning on the oven, see the Christmas fudge recipe that uses cherries, and try that.
Remove from microwave and pour three quaters of the chocolate into a pan covered in foil. Add the mint extract to the rest of the chocolate and if desired add a few drops of food coloring (my mint extract was green, but some come transparent).
Pour it on top of the white chocolate. Chill until firm, about 3 hours, before cutting.