Bring the milk to a boil on the stove, remove it, add 3 tablespoons of powdered sugar and the rum or brandy. Beat the butter with the remaining sugar until it becomes a fluffy cream. Separately, in a container in a water bath, beat the yolks to whitening, being careful not to cook them.
Then add the yolks to the butter and beat again until uniform. Separately, beat the egg whites with a pinch of salt and 1 tablespoon sugar and thoroughly mix with the remaining mixture. Finally, gently mix the cream with the orange peel.
Smear the sides and bottom of a cake form with a bit of the cream. Briefly soak each piece of biscotti in the milk well and arrange in the cake form – a layer of biscotti, layer of cream and so on, until you finish off with the cream.
Make decorations as desired - stick meringues on the top and sides, sprinkle with chopped nuts, biscuits or caramelized sugar.