Boil water and add coffee powder to it.
Let it brew for one minute.
Switch off the flame let it cool down.
Dip Marie biscuits in coffee decoction and keep on a foil.
Spread Nutella on the biscuit.
Keep on stacking 15 biscuits one by one in the same manner.
Spread Nutella on its sides and top also.
If you do not want too much Nutella you can skip this step. but my kids loved it with Nutella smeared on all sides.
Cover with foil and leave in the freezer for 2-3 hours.
Prepare the second log in the same way.
Mocha cake is ready.
Cut diagonally to get beautiful symmetrical lines of Nutella.
Keep in the freezer and serve cold.