In a small saucepan, bring the cream to a simmer. Remove from the heat, and stir in the chocolate and butter. In a medium sized skillet, bring a 1/2 inch of water to a slow simmer. Set the sauce pan in the skillet over low heat. Stir mixture just until the chocolate has completely melted. Remove from the heat.
Pour the chocolate mixture into a shallow bowl. Cool. Cover, and refrigerate until firm, at least 2 hours.
Pour the cocoa into a pie plate. Line an air tight container with waxed paper. Dip a melon baller or a small spoon into a glass of warm water and quickly scrape across the surface of the chilled truffle mixture to form a rough 1 inch ball. Drop the ball into the cocoa. Repeat with the remaining truffle mixture.
Gently shake the pie plate to coat the truffles evenly. Transfer the truffles to the prepared container, separating layers with additional waxed paper. Cover tightly and refrigerate for up to 2 weeks, or freeze up to 3 months.
DIPPING CHOCOLATE FOR TRUFFLES:
10 Ounces Bittersweet Chocolate, Broken Into 1/4 inch pieces
Line a baking sheet with waxed paper. Form the truffles on the prepared baking sheet do not coat with cocoa. Freeze, uncovered, at least 2 hours.
Melt the chocolate in a double boiler over hot, but not boiling, water. Stir the chocolate occasionally until smooth. Remove the truffles from the freezer. Drop one ball into the melted chocolate. Twirl briefly with a fork to coat. Lift the truffle with the fork and drain over the saucepan, return to the baking sheet. Repeat with the remaining truffles. Place the baking sheet into the refrigerator and allow the chocolate coating to set, approximately 1 hour. Place the truffles in a waxed paper lined airtight container. Store up to 2 weeks in the refrigerator or up to 3 months in the freezer.