1 In a medium saucepan, heat the semi-sweet and bittersweet chocolate, butter and milk until the chocolate and butter are partially melted. Remove from heat and stir until completely melted. Whisk in desired flavoring until creamy and smooth.
2 Using a rubber spatula, scrape the chocolate mixture into a glass bowl. Cool to room temperature then refrigerate for about 2 hours to harden.
3 Line a cookie sheet with parchment paper.
4 Using a tablespoon or a spring-action scoop, mold the chocolate into 1-inch / 2.5 cm balls and arrange on the prepared cookie sheet.
5 Place the cocoa powder in a shallow bowl. If necessary, roll truffles by hand to make round; gently toss the truffles 2 or 3 at a time in the cocoa until completely coated. Return truffles to parchment, and allow truffles to harden in the refrigerator for at least 2 hours.
6 Before serving, let stand at room temperature to soften slightly.