8 oz. package of bittersweet/dark chocolate, chopped.  Use the best chocolate you can find.
1 cup whipping cream
4 tablespoons unsalted butter (2 ounces)

Heat the cream gently in a saucepan, stirring until it comes to the boil

Pour over the chopped chocolate and stir gently until the chocolate has melted

Add half the butter, stirring gently till melted and do the same with the remaining butter until all the butter is incorporated

Use as is or place in the fridge for a more solid ganache.  I let it cool to the consistency of thin icing before spreading it, but it will depend on how you need to use it.

Keeps in the fridge for 2 days and the freezer for up to a month.  When it’s a thick liquid, it can be whipped for a fluffy icing, or if it’s really thick, rolled into balls for truffles.

This recipe will make enough ganache to cover a 9” cake, or more than enough for the creampuffs above.

Extra flavours can be added (coffee, mint, etc)

Use a base 1:1 ratio of chocolate to cream (ratio from above), but use less cream (2:1) for a thicker ganache (i.e. truffles) and more cream (1:3) for a thin glaze.  It’s hard to tell how thick your ganache will be when it’s warm, though…  The last time I made it, I used 8 oz of chocolate, and 1/2 cup of milk to make a thicker ganache, and it worked out fine.  It also works fine with semi-sweet or milk chocolate, if you're uncivilized.

This too, is a very traditional recipe.  Do a Google search for other uses, but I'm sure you'll think of your own.  I've used it to top two different cheesecakes, with VERY positive feedback.