1¼ cups bittersweet chocolate, in small pieces
1 cup heavy cream
3 tablespoons butter, cut into small pieces
Put chocolate in a large glass bowl and have butter standing by. Bring cream to a rolling boil. Pour over chocolate. Allow to sit for about a minute, then whisk to blend. Once chocolate is incorporated, toss in the butter pieces and whisk until melted and mixture is smooth. Pour into the cooled pastry shell and chill. (This made enough to fill an 8-inch shell with tall sides plus about half of another 8-inch shell.)
OPTION: Stir in 1/4 teaspoon (or more to taste) of chipotle chile powder as you mix in the butter for a spicy, untraditional ganache delicious in tarts or on yellow cake.