Line a 20 cm (8 inch) springform cake pan with baking paper and butter the sides.
Place the chocolate and butter into a bain-marie (or a double-boiler or a heatproof bowl over a pan of simmering water), and melt over low heat. Set aside to cool slightly.
Using a stand mixer or an electric hand whisk, whisk the eggs and sugar on medium to high speed until the mixture is pale and mousse-like. The mixture should have nearly tripled in volume by this stage.
Slowly mix through the ground almonds, cocoa powder, Amaretto liqueur, and melted chocolate mixture. The consistency of the batter should be similar to a raw chocolate mousse.
Pour the cake batter into the prepared cake pan.
Bake the cake for about 25 minutes, or until a cake skewer inserted in the centre comes out clean. Unfortunately, a cake skewer will leave a hole in your cake and may even cause the top of the cake to crack. I would suggest giving the cake pan a gentle shake - if the centre of the cake wobbles, you will need to bake for another 5 minutes or so. I would use a cake skewer only as the last step.
Let the cake cool in the pan on a wire rack.
Carefully remove the cooled cake to a serving plate or cake stand. The cake is likely to crack during this process, if it hasn't already done so upon cooling.
Generously dust some cocoa powder onto the cake using a fine sieve.
To Make the Whipped Cream
Whisk the double cream with the Amaretto liqueur until it has thickened with soft peaks.
Place the whipped cream into a small serving bowl.
To Serve
Crush the Amaretti biscuits by placing them into a large ziplock freezer bag. Seal the bag and use a large rolling pin to gently crush the biscuits into a mix or small and large crumbs. Place the Amaretti crumbs into a small serving bowl.
Sprinkle some crushed amaretti biscuits over the whipped cream to decorate.
Place a generous spoonful of whipped cream alongside a slice of cake, and allow guests to crumble more amaretti biscuits onto the cream.
It is also not a bad idea to serve some small glasses of Amaretto liqueur alongside.
Kitchen Notes
ALCOHOL SUBSTITUTES If you don't have Amaretto, you could substitute with any other liqueur such as Frangelico, Cointreau, Grand Marnier, or rum. For a non-alcoholic version, simply replace the alcohol in the cake batter with 1 teaspoon of vanilla extract.
USING A LARGER CAKE PAN To make a larger and slightly thinner cake, use a 23 cm (9 inch) springform cake pan and bake for approx 20 mins or until a cake skewer inserted in the centre comes out clean.
MAKE-AHEAD TIPS The cake can be made ahead of time and keeps well on a covered cake stand. The whipped cream can be made a few hours ahead of time and kept covered in the fridge. However, whipped cream is always best when it is freshly made.
OVEN TEMPERATURES All recipes on this website state temperatures for a regular oven (i.e. a conventional oven without fan). If you have a convection oven with a fan, please consult the manufacturer's handbook on how to adjust the temperature and baking time accordingly.