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 1/2 inch of water to a slow simmer. Set the saucepan in the skillet over low heat. Stir mixture just until chocolate has completely melted. Remove from 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 airtight container with waxed paper. Dip a melon baller or 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 truffles evenly.
Transfer truffles to the prepared container, separating layers with additional waxed paper. Cover tightly and refrigerate up to 2 weeks, or freeze up to 3 months.