If chocolate is overcooked in the microwave, the chocolate will seize. This means it will get clumps in it and won't be spreadable. If you are concerned about this, you can melt the chocolate in a double broiler. Stick a heat-safe bowl (preferably metal or a high temperature-safe glass bowl) over a pot of boiling water. Make sure the bowl fits comfortably on top of the pot without falling in. Put the chocolate chips into that top bowl and whisk constantly until melted. Then continue with the rest of the recipe.