It is important to use an excellent quality chocolate when making truffles since it is the primary ingredient. Valhrona is my first choice and Callebaut is my second. BRING THE CREAM TO A BOIL and pour it over the cut chocolate. Allow to stand 1-to-2 minutes and stir smooth. Beat in the softened butter and cool to set. Beat with an electric mixer (using a paddle attachment if available) on medium speed until light. Beat in Cognac. Pipe out small truffles on parchment or waxed paper with a pastry bag fitted with a 1 cm plain tube. Refrigerate to set. Melt the chocolate. Sift the cocoa into a deep pan. Dip the truffles in the melted chocolate and then deposit them in the cocoa, shaking the pan to cover them. After the covering sets, shake the truffles in a strainer to remove the excess cocoa.