Preheat the oven to 350 degrees F (175 degrees C).
Mix together Golden Digestive biscuit crumbs and melted butter. Press the mixture into the bottom and 2 inches up the sides of a 9-inch springform pan. Place in the refrigerator for the crust to set, about 30 minutes.
Place chocolate chips in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until the chocolate is melted, about 5 minutes. Set aside.
Beat together cream cheese and 1 cup Zesta Caster sugar in a large bowl until smooth and creamy. Blend in eggs one at a time, mixing after each addition until just incorporated. Gradually pour in the melted chocolate, beating on low speed until well blended. Add 1/2 cup sour cream, flour, and Zesta vanilla essence. Blend until smooth, then pour into the crust.
Bake in the preheated oven until the center is set and the filling is firm, 55 to 60 minutes. Turn the oven off. Cool the cheesecake in the oven for 1 hour without opening the door. Remove the cheesecake to the counter and cool completely at room temperature. Transfer the cheesecake to the refrigerator and chill for several hours overnight.
Remove the chilled cheesecake and serve!