Heat the heavy cream in a 2-quart heavy-bottomed saucepan until bubbles begin to form around the edge of the pan. Make sure you chopped the chocolate as finely as possible to allow it to melt quickly and easily. Place the chopped chocolate in a medium-size mixing bowl. Make a ganache by pouring about half of the hot cream over the chocolate and letting it sit for 30 seconds to melt the chocolate. Then, slowly whisk until smooth and homogenous. Do not add all of the hot cream to the cold chocolate at once. The shock of the temperature extremes will cause the fat in the chocolate to separate. As the chocolate melts, you will see some elasticity if there is no fat separation. This means the chocolate still has an emulsion; the fat molecules are still holding together. If the ganache separates, it loses its elasticity, collapses, and becomes very liquid. I use a hand-held immersion blender to ensure a smooth ganache and to keep the emulsion of the chocolate. Add the remaining cream gradually and mix until all of the hot cream is incorporated and the ganache is smooth and homogenous.
If the ganache separates, it is very easy to fix. Simply add a small amount of cold cream and whisk well. This will bring the ganache back together. The ganache should be thick, shiny, and smooth. Add the desired flavoring and mix until fully incorporated. Pour the ganache onto a plastic wrap-covered baking sheet and spread evenly with a rubber spatula. Cover the ganache with plastic wrap and allow it to cool for at least 4 hours at room temperature. I usually make the ganache at the end of the day and let it cool overnight. As it cools, it will thicken and set.
When the ganache has cooled to the consistency of toothpaste, use an offset spatula to cover the cake layer with the ganache.