Make the Crust:

1. Preheat oven: Preheat oven to 160°C or 325°F.
2. Crush biscuits: Crush the digestive biscuits into fine crumbs using a food processor or by placing them in a ziploc bag and bashing them with a rolling pin.
3. Combine ingredients: Pour the crumbs into a bowl and add the melted butter. Mix until well combined.
4. Press into pan: Press the crumb mixture firmly into the bottom and up the sides of a 9-inch springform pan. Set aside.

Make the Cheesecake:

1. Beat cream cheese: In a large bowl or the bowl of a stand mixer, lightly whisk the cream cheese until creamy and smooth. Avoid overbeating.
2. Add sugar: Add the caster sugar and mix until well combined.
3. Add remaining ingredients: Add the sour cream, vanilla extract, and salt. Stir until well combined, scraping down the sides of the bowl as needed.
4. Add eggs: With the mixer on low speed, slowly add the beaten eggs, a little at a time, stirring just until combined.
5. Pour batter: Pour the cheesecake batter into the prepared springform pan.
6. Bake: Place the cheesecake pan on a baking sheet lined with tin foil. Bake at 160°C or 325°F for 50-60 minutes, or until the edges are slightly puffed and golden brown and the center springs back to the touch but is still jiggly. Do not overbake.
7. Cool: Remove from the oven and let the cheesecake cool on top of the oven for 10 minutes. Then, use a knife to gently loosen the crust from the inside of the springform pan to prevent cracking. Let it cool completely in the pan.
8. Chill: Once cooled, cover the cheesecake and chill in the refrigerator overnight or for at least 4 hours.

Make the Crème Brûlée Topping:

1. Remove cheesecake from pan: After chilling, remove the cheesecake from the refrigerator and carefully remove the ring from the springform pan.
2. Sprinkle with sugar: Evenly sprinkle the granulated sugar over the top of the cheesecake.
3. Caramelize: Use a kitchen torch to caramelize the sugar until it forms a golden brown crust.
4. Serve immediately: Let the caramelized sugar cool slightly before slicing and serving.