Caramelize the sugar in a suitable container and mix it with the water, walnuts and rum essence. Lay out the mixture onto a baking sheet, smooth it out, allow it to cool, and crumble it with a mallet. Divide into 2 parts.
For the Swiss buttercream, beat the egg whites and powdered sugar with a mixer in a water bath (on low heat). The water must not come to a boil, continue beating for about 4 minutes.
Transfer the beaten whites into a dry bowl and finish beating with the mixer on high until you get a fluffy, hard snow. Add the rum essence, and while constantly beating, add the butter in 4-5 parts, mixing well after each one.
After you add the final bit of butter, continue beating for another 10-15 minutes until the cream thickens and becomes smooth and dense. Make the cream just before you start assembling the cake, so it doesn't harden. Set aside 1/3 of it to cover the cake.
Lay out a cake form with removable sides with baking paper. You have to make a total of 5 layers of biscuits with cream and caramelized sugar; be sure to dip the biscuits in the milk. Alternate between a layer of biscuits, buttercream, caramelized sugar and sprinkle with natural cocoa. After the last layer, cover with foil and leave in the fridge to firm up for two hours.
Then remove the sides of the form and distribute the remaining 1/3 of the cream over the entire cake evenly. Immediately cover with remaining caramelized sugar and make a cocoa border around the cake. Form the decoration by making equal lengths of cocoa strips, using them to outline 12 identical cake slices.
Leave the biscuit cake with Swiss buttercream to harden in the fridge for a little while longer before serving.