Place the chocolate pieces in a large heat-proof mixing bowl.
In a 1.5-qt. sauce pan, bring the cream to a simmer on medium heat. Add the butter and vanilla.
Pour the heated cream over the chocolate and let stand for a few minutes.
Whip the cream and chocolate together until smooth and shiny.
Add the sour cream and cognac and combine until smooth.
Pour the truffle mixture into a shallow bowl. Cover the surface with a piece of plastic wrap and refrigerate until thoroughly chilled (2 - 3 hours).
Scoop 1-oz. balls of the mixture and drop into the cocoa powder. Roll the truffles into irregular balls in the palm of your hand.
Place the truffles in individual candy cups and then into an airtight container and refrigerate.