In a bowl with a fork stir together the crumbs and the butter until the mixture is combined well, pat the mixture onto the bottom and 1 inch up the side of a lightly oiled 8-inch springform pan, 2 1/2 inches deep, and freeze the crust for 30 minutes, or until it is firm. Spread the ice cream, softened, evenly on the crust and return the pan to the freezer for 30 minutes, or until the ice cream is firm. In a bowl with an electric mixer beat the cream with the vanilla until it holds stiff peaks, fold in the amaretti thoroughly, and spread the mixture over the ice cream. Smooth the top of the cake, sprinkle it with the almonds, and freeze the cake for 30 to 45 minutes, or until the top is firm. Freeze the cake, covered with plastic wrap and foil, for at least 4 hours or overnight.
For the sauce: In a small heavy saucepan combine the cream and the brown sugar, bring the mixture to a boil over moderately high heat, whisking occasionally, and whisk it until the brown sugar is dissolved. Remove the pan from the heat, add the bittersweet chocolate and the unsweetened chocolate, and whisk the mixture until the chocolate is melted. Whisk in the butter and the Amaretto until the sauce is smooth and let the sauce cool slightly. The chocolate sauce may be made 1 week in advance and kept covered and chilled. Reheat the chocolate sauce over very low heat, stirring occasionally, until it is warmed.
Just before serving wrap a warm dampened kitchen towel around the side of the pan, remove the side, and transfer the cake to a serving plate. Cut the cake into wedges with a knife dipped in hot water and serve it with the chocolate sauce.