1. Prepare the Crust:
* In a food processor, pulse the digestive biscuits or graham crackers until they form fine crumbs. Alternatively, place them in a ziplock bag and crush them with a rolling pin.
* Transfer the crumbs to a medium bowl. Add the optional granulated sugar (if using) and stir to combine.
* Pour the melted butter over the crumbs and mix thoroughly until all the crumbs are moistened and the mixture resembles wet sand.
* Press the crumb mixture firmly and evenly into the bottom of a 20-23cm (8-9 inch) springform pan. Use the bottom of a glass or a measuring cup to create a compact, level base.
* Chill the crust in the refrigerator for at least 30 minutes (or in the freezer for 15 minutes) while you prepare the filling. This helps it firm up.

2. Prepare the Matcha Mixture (If using Gelatin/Agar-Agar):
* If using gelatin or agar-agar for extra stability:
* For Gelatin: Sprinkle the gelatin powder over 3 tablespoons of cold water in a small, heatproof bowl. Let it sit for 5-10 minutes to bloom (it will become jelly-like). Then, gently heat this mixture (either in a microwave for 10-15 seconds or over a pan of simmering water) until the gelatin is completely dissolved. Do not boil. Let it cool slightly.
* For Agar-Agar: Whisk the agar-agar powder with 3 tablespoons of cold water in a small saucepan. Bring to a gentle simmer over medium heat, whisking constantly, and cook for 1-2 minutes until fully dissolved. Remove from heat and let it cool slightly (it will thicken as it cools, so don’t let it set completely).
* In a small bowl, sift the matcha powder to remove any lumps. Add 2-3 tablespoons of hot (not boiling) water or a small amount of the unwhipped heavy cream (about 2 tablespoons) and whisk vigorously (a bamboo matcha whisk or a small regular whisk works well) until you have a smooth, lump-free paste. Set aside.

3. Make the Cheesecake Filling:
* In a large mixing bowl, beat the softened full-fat cream cheese with an electric mixer on medium speed until smooth and creamy, with no lumps. This might take 2-3 minutes. Scrape down the sides of the bowl as needed.
* Add the sifted icing sugar to the cream cheese and beat on low speed until just combined, then increase to medium speed and beat until smooth and fluffy.
* Add the vanilla extract, optional lemon juice, and the prepared matcha paste to the cream cheese mixture. Beat on low speed until everything is well incorporated and the mixture is a uniform green colour. Be careful not to overmix.
* If using dissolved and slightly cooled gelatin/agar-agar, slowly pour it into the cream cheese mixture while the mixer is on low speed, mixing until just combined.

4. Whip the Cream:
* In a separate, clean, and chilled bowl, whip the cold heavy whipping cream with an electric mixer (using clean beaters) on medium-high speed until stiff peaks form. Stiff peaks mean that when you lift the beaters, the cream holds its shape and the peaks point straight up or curl over slightly without collapsing.

5. Combine and Assemble:
* Gently fold about one-third of the whipped cream into the matcha cream cheese mixture using a spatula to lighten it.
* Then, add the remaining whipped cream and continue to fold gently until just combined and no white streaks remain. Be careful not to deflate the whipped cream – the goal is to maintain its airiness.
* Pour the matcha cheesecake filling over the chilled crust in the springform pan.
* Smooth the top with an offset spatula or the back of a spoon for an even surface.

6. Chill to Set:
* Cover the springform pan loosely with plastic wrap (try not to let it touch the surface of the cheesecake).
* Refrigerate for at least 6-8 hours, or preferably overnight, until the cheesecake is firm and well set. If you’ve used gelatin or agar-agar, it might set a bit faster.

7. Serve:
* Once fully chilled and set, carefully run a thin knife around the edge of the cheesecake to loosen it from the sides of the springform pan before releasing the spring.
* If desired, dust the top with a little extra matcha powder (using a fine-mesh sieve) or decorate with whipped cream, fresh berries, or white chocolate shavings just before serving.
* Slice with a sharp knife, wiping the knife clean between cuts for neat slices.