3 parts semi-sweet chocolate
2 parts hot, not boiling, heavy cream
Dash of vanilla extract, optional
Melt three parts semi-sweet chocolate in a double boiler or in the microwave, then slowly add two parts not-quite boiling, hot heavy cream. Remove from heat, but allow the mixture to rest for a few minutes before stirring together. The mixture might appear thick and clumpy, but continue to stir until smooth. When the ganache is well blended, add the optional vanilla (or other) extract. Use about one teaspoon per cup of chocolate.
When choosing chocolate, remember that white chocolate is not actually chocolate, so it will be more difficult to work with initially. Also, milk chocolate has higher sugar content than darker chocolates, so it will result in a thinner ganache.
Also, adjust the ratio of chocolate to cream depending upon your intended use. If you need a thick ganache, add more chocolate and a touch less cream, but if you are making a glaze rather than a filling or frosting, be generous with the cream and cut back just a little on the chocolate. Note that ganache thickens as it cools.
This recipe can be used for decorative glazes, cake frostings, candy fillings, and as a topping.