Mix the biscuit crumbs, 100g sugar and melted butter together in a bowl. Press mixture evenly into the bottom of 23cm round springform tin.
Melt 85g dark chocolate chips in a pan over low heat while stirring constantly. Set aside.
Beat the cream cheese, sugar and cornflour together in a mixing bowl until smooth. Slowly beat in the eggs and egg yolk, one at a time, until thoroughly blended. Stir in the orange juice, orange extract, orange zest, orange food colouring, if desired and vanilla. Reserve 225g of the cream cheese mixture and set aside. Pour the remaining mixture over the prepared base. Stir the melted chocolate chips into the reserved 225g cream cheese mixture. Pour the chocolate mixture into the orange filling and swirl with a knife.
Bake in preheated oven for 15 minutes. Lower heat to 105 C / Gas 1/4 and bake until centre springs bake when touched, 75 to 85 minutes. Turn the oven off and open the oven door; allow cheesecake to cool in the oven at least 30 minutes. Remove from the oven and refrigerate, uncovered, for at least 8 hours or overnight.
Remove the cheesecake from the springform tin by running a knife around the inside edge. Release the side of the tin and lift off. Run a knife under the bottom of the cheesecake and carefully slide onto a serving plate.
To make the pecan-chocolate topping, melt 4 tablespoons dark chocolate chips in a pan over low heat while stirring constantly. Remove from heat and stir in the golden syrup and soured cream until smooth and evenly blended. Drizzle over the top of the cheesecake. Garnish with pecans.
To make the orange topping, stir the cornflour and water together in a small dish to make a smooth syrup. Place the sugar and orange juice in a pan and bring to the boil over medium-high heat. Reduce heat to medium and stir in the cornflour mixture; cook until thick and clear. Cool slightly and drizzle over the top of the cheesecake.