In a large mixing bowl, beat the butter with a hand mixer on low speed until light and creamy.
Sift in the icing sugar and cocoa. Stir gently to combine (so it doesn't fly everywhere), before continuing to beat with the hand mixer until smooth. Taste as you go because you may want to add a bit more sugar or cocoa.
Add the salt and whipping cream and beat again until light and fluffy.
If the frosting now seems too soft, keep it in the fridge for 10 minutes.
Keep your biscuits ready along with the platter or tray on which you plan to assemble the cake.
Mix the coffee and water together in a small bowl until smooth.
Working with one biscuit at a time, dip each one into the coffee solution for just a couple of seconds, without turning it over. Place it on the plate. It will continue to soak and soften as it rests. I made 7 layers of 10 biscuits each (therefore 70 biscuits total), with five biscuits in each row. Repeat the dipping till the first layer of soaked biscuits is ready.
Now spread about 1/3 cup of the frosting in an even layer on the biscuits. Make a second layer of soaked biscuits on top of the frosting. Repeat till all the biscuits and frosting are used up.
Smoothen the sides with the frosting that would be seeping out through the layers, and spread more on as needed till the cake is completely covered. If you're working in a warm kitchen, you may have to chill the cake in-between layers.
Top with chopped or grated chocolate and chill the cake for at least 4 hours before cutting into pieces. I like it best when it's been at room temperature for about 10 minutes. But at all other times, keep it refrigerated in an airtight tin, and it'll be good to eat for 4 to 5 days. Remember that the biscuits will soften the longer the cake is stored. Happy eating!