First, heavy cream and heavy whipping cream are the same thing.
If you are adding butter you are not making a true ganache, the butter will make your final product softer, more like an icing or a glaze.
This was actually hit on in the first post: You are trying to use a product (chocolate chips) that have added junk in them to make them hold their shape under high heat so they won't melt. Read the ingredients! Sure, you can make them work, or you can save yourself the headache and buy natural chocolate that is meant to be melted.
Do not melt your chocolate first, especially since you are probably burning it and didn't even know it. That completely goes against chocolate science. When making a ganache you are tempering it. Semi sweet chocolate melts between 86-90 degrees, your perfect melting temperature is 98. You do not want your chocolate to go over 98 degrees thru this whole process, which is why you pour hot cream over your chocolate.
The higher sugar content and lower cocoa content of your chocolate, the easier it is to burn, causing lumpy chocolate.
To make ganache:
Run your chocolate thru a food processor to break it down or finely chop with a knife.
Bring your cream to just a boil, pour it over the top. Do NOT cover with plastic wrap. This can cause condensation on the plastic, dropping water in your ganache and making it seize (lumpy). Let sit for 5 minutes then stir. Use a silicone spatula.
If you still have chunks of unmelted chocolate, do not microwave, you can seize your chocolate or burn it because it's pretty impossible to keep the temperature under 98 degrees in the micro.
The best way to make perfect ganache is get a heating pad, put it in a big bowl, turn it on high and put your bowl of ganache on it. Let it sit for 20-30 minutes, gently stirring occasionally until smooth. Take your bowl off and let it cool to the consistency you want it, then put it back on the heating pad while you work with it. It will help prevent the chocolate form getting too cold cold to work with.