1.To make caramel sauce, stir sugar and the water in a medium saucepan over medium heat until sugar dissolves, brushing down side of pan occasionally with a wet pastry brush. Bring to the boil. Reduce heat; simmer, without stirring, for 7 minutes or until a deep caramel colour. Remove pan from heat. Carefully add cream, taking care as mixture will splutter. Return to heat; stir until smooth. Cool caramel until just warm. Gradually whisk in butter until combined. Transfer sauce to a jug, cover; set aside at room temperature until required.
2.Meanwhile, to make custard, heat milk and cream in a large saucepan until almost boiling. Whisk egg yolks and sugar in a large bowl until combined. Gradually whisk milk mixture into yolk mixture. Return mixture to pan; stir constantly over low heat for 7 minutes or until custard thickens. Do not boil. Strain mixture into a large bowl; stir in liqueur. Cover surface with plastic wrap; refrigerate 4 hours or until chilled.
3.Grease a 22cm (9-inch) springform pan; line base and side with baking paper. Reserve 1 tablespoon amaretti crumbs to serve. Sprinkle one-third of the remaining amaretti crumbs on base of the pan. Place pan in freezer to chill.
4.Churn half the chilled custard in an ice-cream machine according to manufacturer’s instructions. Working quickly, spoon ice-cream into chilled pan; smooth surface. Using a quarter of the caramel sauce, dollop 4 tablespoons onto the ice-cream layer. Using a knife, gently swirl the caramel through ice-cream. Sprinkle with half the remaining amaretti crumbs. Cover with plastic wrap; freeze for 1 hour or until firm.
5.Repeat step 4 with remaining chilled custard, another quarter of the caramel sauce and remaining amaretti crumbs. Cover with plastic wrap; freeze overnight until firm. Stir extra amaretto into caramel sauce; refrigerate until required.
6.Before serving, reheat remaining caramel sauce in a microwave oven on low. Drizzle sauce over ice-cream cake; sprinkle with reserved amaretti.
You will need an ice-cream machine and you’ll need to start this recipe the day before. This recipe only uses egg yolks, which of course means you will have 18 egg whites left over. The good news is that you can freeze them for later use. As you separate the eggs, place each white into the hole of an ice-cube tray. Once frozen transfer them to a small container or resealable plastic bag, and note the date; they will keep for up to 1 year. Thaw in the fridge before use.