Step 1Microwave chocolate at half power to melt completely, stirring a few times to ensure even melting, 2 to 3 minutes.
Step 2Heat cream in saucepan over medium-low heat just to a simmer, but not boiling, 3 to 4 minutes. Pour over melted chocolate and stir from center to mix evenly.
Step 3Add butter and zest and stir until thoroughly combined.
Step 4Cover and refrigerate 3 hours. This will allow any remaining air bubbles to work their way out.
Step 5Using small ice cream scoop or tablespoon, scoop truffle mix and form into round balls using your hands; work quickly, because the chocolate will begin to melt as it warms. (Latex gloves are recommended.) Roll each truffle in cocoa powder and store in refrigerator until serving.