Pour the cold coffee and amaretto or brandy, whichever you're using, into a bowl. Add half the amaretti biscuits and allow them to soak for a couple of minutes.
Meanwhile, place the egg yolk, the caster sugar and the mascarpone in a separate bowl and whisk them together for 3-4 minutes, or until they are light and fluffy. In another spotlessly clean bowl, whisk the egg white until it forms soft peaks, then carefully fold it into the mascarpone mixture.
Divide the soaked amaretti biscuits between the glasses or bowls. Next, divide approximately half of the egg-white-and-mascarpone mixture between the glasses or bowls, then sift a little of the cocoa powder into each one.
Repeat with the remaining ingredients to make a second layer of amaretti biscuits, and egg-white-and-mascarpone mixture. Serve immediately or keep covered in the fridge for several hours or overnight until you're ready to serve it.