In a large chilled bowl, break up ice cream with a spoon, then quickly mix in spices. Drizzle and fold in molasses or treacle until blended. Freeze for 30 mins or until firm.
Meanwhile, line the base and side of a 22cm springform pan with baking paper, allowing the side to extend by 1cm. Arrange 12 biscuits over the base to cover, trimming if necessary.
Remove ice cream from freezer. Quickly spread one-quarter of the ice cream evenly over the biscuits. Continue layering with remaining biscuits and ice cream, finishing with ice cream. (If necessary, freeze cake for 30 mins after each layer, keeping remaining ice cream frozen in the bowl and mixing it before spreading.) Smooth the top of the cake and freeze for 12 hours or until firm.
Meanwhile, to make mini gingerbread biscuits, whisk the flour, spices and bicarbonate of soda in a bowl. In an electric stand mixer on medium-high speed, beat butter, sugar and treacle for 6 mins or until fluffy. Beat in the egg. Beat in the flour mixture on low speed. Divide into 2 portions. Roll out each portion between 2 sheets of baking paper until 3mm thick. Chill for 1 hour. Use a 5cm gingerbread cutter to cut out shapes. Chill for 1 hour or until firm.
Preheat oven to 160°C (140°C fan-forced). Bake the gingerbread on lined trays for 15 mins or until golden. Cool on the trays for 10 mins. Cool completely on a rack.
Cover surface of cake with plastic wrap. Invert onto a plate. Remove pan and paper. Invert onto a serving platter and freeze for 30 mins.
Meanwhile, line a baking tray with baking paper. In a small bowl, stir the chocolate and oil to blend. Cool slightly. Pour ⅓ cup (80ml) chocolate mixture onto the tray and shape into a 25cm disc. Freeze for 30 mins or until set. Break into pieces and keep frozen. Keep remaining chocolate mixture at room temperature.
To serve, spoon whipped cream over the centre of the cake. Top with the chocolate pieces and gingerbread. Drizzle with a little chocolate mixture. Cut into wedges. Serve immediately with remaining chocolate mixture.