If using a block, chop into small pieces.
In a microwave or on a small pot, bring to a boil the cream+ butter.
Add the hot cream to the chocolate.
Using a spoon start to slowly stir the chocolate.
Stirring should not be fast to avoid incorporating air into the ganache.
It's ready when no more cream is visible.
Allow to cool to room temperature before using to get best results. You can refrigerate or freeze after use. You can also add a little brandy/whiskey to flavor your ganache.
Use to decorate your birthday cake......
Chocolate banana cake, check one of my other guides for the cake recipe.
Another example. Butter helps in attaining the glossy finish.