Whack cookies in ziplock bag with rolling pin till you have smallish chunks, do not powder them, rather bits and pieces.
Place in medium bowl and set aside.
Place both kinds of chocolate in microwaveable bowl together with butter/marg.
Zap for a minute, check and zap another minute and stir well till smooth and glossy.
Immediately add corn syrup and heavy cream. Stir again till smooth.
Pour over the whacked cookies in the bowl and stir till all the bits and pieces are coated.
(You will see it's still got some pockets of liquidy chocolate. That's correct, no worries, it will harden up and suspend the cookies in the chocolate).
Pour mixture into a parchment lined long loaf pan or 9 x 9 (23 x 23 cm) pan.
Place in fridge for at least 2 hours
Remove from fridge, flip onto platter, peel off parchment. Chocolate will have hardened. Best kept in fridge till serving. Let sit about 20-30 minutes before slicing and serving.
