Separate the eggs.
In a large mixing bowl, whisk the egg yolks together with the caster sugar until the sugar is entirely melted and the yolks have taken on a light, frothy texture.
In a separate bowl, whip the egg whites with heaping spoonful of the confectioner's sugar until they form stiff peaks.
In yet another bowl, whip the cream with a heaping spoonful of the confectioner's sugar until it, too, forms stiff peaks.
Now fold the crumbled amaretti into the whisked yolks, along with the liqueur if using, with a spatula.
Add the whipped whites and cream into the bowl with the egg yolk mixture, folding them, too, into the mixture very gingerly. Use a circular motion, scooping up from the bottom of the bowl until you have a uniform batter.
Line a loaf pan with plastic wrap (cling film) and pour in your batter. Smooth out the top as much as you can, then lay some plastic wrap over the top.
Place the loaf pan in the freezer and leave it for at least 4-5 hours, or overnight.
Uncover and then unmold your semifreddo on to a serving plate. Gently remove the plastic wrap. Cover the top of the semifreddo with a layer of the sliced almonds, pressing them gently so they stick to the surface.
Let the semifreddo rest for about 15-20 minutes before slicing to serve.