Prepare the butter cream by mixing the egg, butter, margarine and the icing sugar. Put the mixture in two bowls, adding the Nutella only to one bowl. Prepare two bowls containing equal parts, with cognac and coffee add sugar, and Maraschino and Amaretto. Moisten the dish where you will place the semi-freddo with cold coffee. Quickly dunk the Osvego biscuits in cognac/coffee and arrange on the plate. Then cover with a layer of the Nutella butter cream. Make a layer with the Osvego biscuits, dunked in the Maraschino/Amaretto and then cover them with the butter cream. Alternate layers. Make sure you finish the cake with the Nutella cream layer. Put it in the freezer for at least 4 hours. Pull it out of the freezer at least 30 minutes before serving.