Preheat oven to 160°C. Line the base of a round 20cm (base measurement) springform pan with baking paper.

Place the biscuits in the bowl of a food processor and process until they resemble fine breadcrumbs. Add the butter and process until well combined. Place in the prepared pan and use a straight-sided glass to spread and press the biscuit mixture on the base and side. Place in the fridge for 30 minutes to chill.

Meanwhile, place the cream cheese and sugar in a food processor and process until smooth. Add the eggs, one at a time, processing well between each addition. Add the sour cream and thickened cream and process until combined. Pour evenly among two medium bowls. Add the Grand Marnier and orange rind to one portion and stir to combine. Add the chocolate to the remaining portion and stir to combine.

Pour half of the chocolate mixture in the prepared base. Pour half of the orange mixture over the top. Continue layering with remaining orange and chocolate portions. Use a bamboo skewer to gently swirl the mixtures to create a marbled pattern.

Bake for 45 minutes or until just set. Turn off the oven and set aside for 2 hours to cool (see note). Transfer to the fridge to chill completely. Top with glace orange slices, if desired. Cut into wedges to serve.