Bake the chocolate cake according to package directions in two 8x8 or 9x9 cake pans. Cool completely.
When the cakes are cool, and you're ready to compile the first layers, soften the ice cream in the microwave. You want it soft enough to spread, but not so much it's runny. Trim the first chocolate cake layer as needed (I always slice the top off to get an even surface) and place it on a cake stand. I put the side I just trimmed face down, so the smooth side is on top. Using a spatula, cover the first layer with the softened ice cream, going not quite to the edges. If you go all the way to the edges, it will squish over when you add the top layer. I like to put a thick layer of ice cream, but you can put as much or as little as you like!
Trim the top of the second layer so it's flat, then place that side down on top of the ice cream layer. Place in the freezer until the ice cream is solid, and you're ready for the ganache layer.
When you're ready to add the ganache, melt the chocolate chips and heavy cream in a glass bowl in the microwave, just until the chocolate chips are starting to melt--about a minute. (You can also use a double boiler if you prefer.) Stir with a spoon until it's glossy. Use a large spoon to add it to the top of the cake, bringing it all the way to the edges. It's okay if it spills over the sides a little--it looks yummy that way! :)
Freeze until about 15 minutes before you're ready to serve--take it out of the freezer and let it soften enough to slice into pieces.