To prepare with a double boiler (preferred method):
Heat a medium saucepan with approximately 2-3 inches of water over medium high heat. Place a metal bowl on top of the saucepan and add the chocolate, heavy cream and vanilla extract.
As the water simmers and turns to steam, gently stir the ingredients together as they melt. If it looks like the sauce is getting too hot, remove the saucepan from the heat and allow the sauce to continue melting over the residual steam. They key is to melt the chocolate slowly.
To prepare in the microwave:
Place the chocolate, heavy cream and vanilla in a microwave safe dish.
Microwave for 1 minute on medium (50%) power. Remove from the microwave and stir until smooth.
Feel free to substitute semi-sweet or milk chocolate in place of the bittersweet chocolate. White chocolate is trickier to temper so I do not recommend using it in this recipe.For a boozy twist, try swapping out the vanilla for red wine, Chambord or Kahlua!Chocolate can seize up if it comes into contact with water or it gets too hot. This is a great article with more details. The microwave can be a bit finicky but should be safe for melting the chocolate at 50% power.