In a large bowl, sift together confectioners' sugar and cocoa; set aside.
In a small saucepan, melt candy bars and butter. Add cream and reserved cocoa mixture. Cook and stir over medium-low heat until mixture is thickened and smooth. Pour into an 8-in. square dish. Cover and refrigerate overnight.
Using a melon baller or spoon, shape candy into 1-in. balls; press a hazelnut into each. Reshape balls and roll in ground hazelnuts. Store in an airtight container in the refrigerator.