In a small saucepan, simmer the heavy cream over a low heat.
Place chopped chocolate in a separate bowl. Add the heavy cream and vanilla and let stand for a few minutes to melt the chocolate. Stir until smooth.
Let cool. Place bowl in the refrigerator for 2 hours.
When the chocolate mixture has solidified, use a teaspoon to roll out 1-inch balls. Roll them in your hands quickly to give them an even shape. Place rounded balls on a baking sheet lined with wax paper and let sit in the refrigerator for 8 hours (or overnight). Roll the chocolate balls in cocoa powder until evenly coated.
Keep the truffles refrigerated until ready to serve!