In a large bowl, sift together the confectioners' sugar and cocoa; set aside.
In a saucepan, melt the candy bars and butter.
Add the cream and reserved cocoa mixture.
Cook and stir over medium-low heat until the mixture is thickened and smooth.
Pour into an 8-in.square dish.
Cover and refrigerate overnight.
Using a melon bailer 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.