Heat the cream in a saucepan over low-medium until it's steaming but NOT boiling (145℉/63℃). Turn off the heat.
Add the chopped chocolate to the steaming cream. Cover the pan to keep the warmth in. Let it sit for a few minutes for the chocolate to melt. This should take 2 to 3 minutes, depending on the size of the chocolate pieces. Then, stir it all together until it's smoothly blended.
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Notes
Use a good-quality baking chocolate that melts well here, rather than chocolate chips.
Chop the chocolate as finely as possible so that it will melt completely and evenly.
Use heavy whipping cream, not half-and-half or anything lighter.
Chocolate ganache will thicken as it cools. If you want to drizzle it over a cooled cake so that it runs down the sides, use it right away while the ganache is still warm. If you want to spread it out thicker over a cake layer, allow it to cool more.
To make whipped chocolate ganache, allow it to cool at room temperature for two hours. It should be about the consistency of peanut butter. Then, whip it for 2 to 3 minutes, until it becomes lighter in color and texture and will hold stiff peaks. Use it to frost cupcakes or frost and fill a layer cake.
Leftovers will keep in the refrigerator for a few days.
If you must gently reheat it to use it again, stir it constantly over low heat, being careful not to let any water in. Water added at this point can cause the chocolate to seize.