In a medium heat-proof bowl, place the chopped bittersweet and milk chocolate. Set aside. In a small saucepan over medium heat, bring the heavy cream and butter to a low boil. Remove from the heat and pour the mixture over the chocolate. Let stand for 1 minute. Using a rubber spatula, stir mixture until melted and smooth. Cover and chill in refrigerator for at least 4 hours or until firm.

Place cocoa powder in a small bowl. Take chocolate mixture out of the fridge. Using an ice cream scoop, spoon, or melon baller, scoop out a section and form into 1-inch balls with your hands. Roll each ball in cocoa coating.

Store truffles in fridge in an airtight container for up to a week.

Truffles can be frozen for up to a month. To thaw, leave them in the fridge overnight.