Into a separate bowl, crush biscuits by hand into roughly 3-4cm pieces. Set aside.
Place chocolate buttons in a large mixing bowl and set aside. Add diced unsalted butter and thickened cream to a medium saucepan and melt over medium heat. Pour the hot cream mixture immediately over the chocolate buttons and mix well until they have melted. Add sifted icing sugar and Chocolate Spice into the chocolate mixture and whisk thoroughly to combine.
Add crushed biscuits and mix with a wooden spoon until all biscuit pieces are generously coated with chocolate. Place mixture into the fridge to semi-set for 20 mins.
Prepare a piece of baking paper approximately 60cm in length and place it on a bench. Take the semi-chilled biscuit mixture out of the fridge. Using a spoon, place spoonfuls of the mixture along the longest length of the baking paper (the edge closest to you) to form a log shape but leave a 15cm gap at either end. Gently roll up the log in the baking paper so it is fully encased and carefully twist the ends as tight as you can.
Next, take a large sheet of aluminium foil and wrap it firmly around your Mosaiko log, twisting the ends of your wrapper. Gently roll your wrapped Mosaiko log back and forth on the bench to smooth its shape. Place in fridge for a minimum of 5 hours to set or, preferably, overnight.
Remove from the fridge when ready to serve. Unwrap Mosaiko and place it on a platter. Sprinkle with a dusting of icing sugar if desired. Cut into 2cm slices to serve.