Even though you are adding coffee powder, there won’t be any coffee flavor. It only enhances the chocolate flavor.
You can use any flavor ice cream you like, but what would go well with the cake according to me are pralines n cream, cookies n cream or vanilla flavor.
Whipped cream can be used as a topping too.
Make sure you push the chocolate deep into the cake until it is fully covered. I didnt do that so, it was seen as a dent on top after baking.
Please note that the cooking time will depend on the microwave wattage. The key for perfect mug cake is that you dont overcook the batter. The cake should rise a bit, ideally with a fluffy, moist, gooey texture and glossy top. The more you will cook, the more the cake will harden up and dry-so slightly undercook and keep an eye on them all the time.