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Place the chocolate in a medium heat-proof bowl.
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Put the heavy cream in a small saucepan set over medium heat and warm the cream until it begins to steam—don’t let the cream boil.
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Pour the hot cream over the chopped chocolate and allow to sit for 3 minutes. After 3 minutes, stir gently IN ONE DIRECTION until the mixture is smooth. Add the champagne and butter and stir until well combined.
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Let the mixture sit at room temperature for 45 minutes, then cover with plastic wrap and refrigerate for 4 hours, or up to overnight. (You can store the ganache in Tupperware in the fridge for up to 2 weeks.)
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After 4 hours, the ganache should be set enough that you can scoop it into balls; use 1 heaping teaspoon of ganache per ball. Put 1/3 cup of cocoa powder into a shallow bowl.
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Roll each ball of ganache between your palms to shape it, then roll in the cocoa powder.
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Uneaten truffles can be stored in an airtight container in the fridge for about 2 weeks, or in the freezer for about 2 months.