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1
Put the chopped chocolate in a small heatproof bowl.
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2
Bring the cream to a boil (you can do this in a microwave oven) and pour it over the chocolate. Let it sit for 30 seconds and then, using a whisk or heatproof spatula, gently stir the chocolate and cream together in small circles, starting in the center of the bowl and working your way out in ever-widening concentric circles. When the ganache is smooth, add the butter one piece at a time, stirring until it is incorporated.
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3
If you're using the ganache as a filling for a tart or glaze, use it immediately. If you're using it to fill and frost a cake, you'll have to wait for it to thicken. You can leave it on the counter, stirring occasionally (it thickens slowly), or you can set the bowl into a larger bowl filled with ice cubes and water, in which case, stir often and stay close -- it thickens quickly. Alternatively, you can put it in the refrigerator, checking on it and stirring frequently. If you miss the moment, you can always reheat the ganache (see below).
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4
Storing: The ganache can be covered tightly and kept in the refrigerator for up to 5 days or frozen for up to 2 months. You'll have to bring it back to the consistency you need before using it, either by leaving it out at room temperature or warming it. Heat it in 5-second spurts in a microwave or put it in a bowl set over a pan of simmering water. With ganache (especially white chocolate ganache), the keys to success are very low heat and a very light touch.
