Line the bottom of a 20cm (8") springform pan with parchment paper. In a large bowl, mix together digestive biscuits/graham cracker crumbs, butter, and sugar. Using a bottom of measuring cup or flat-bottom glass, press tightly into the bottom of the prepared pan. Put into the fridge while you make the filling.

For the filling, in a small saucepan mix gelatin with 2 TBSP water and 2 TBSP lemon juice. Set aside for 10 minutes.

Heat the gelatin over low heat until dissolved, but do not boil.

In a large mixing bowl, add plain yogurt, sugar, vanilla extract, and the rest of the lemon juice. Beat until fully incorporated.

In another mixing bowl, beat heavy cream until stiff peaks form.

Add a few tablespoons of yogurt mixture into dissolved gelatin and stir until combined.  Pour back to the yogurt mixture and mix until combined.

Using a rubber spatula, gently fold in whipped cream, until fully combined.

Take the springform pan from the fridge and arrange the raspberries on top of the prepared crust. Using a spoon, spoon prepared filling over the crust, until the raspberries are covered (if you just pour the filling over, the raspberries will get pushed to the sides of the pan), then pour the rest of the filling.

Put into the fridge for at least 5 hours.

For the raspberry sauce/jelly, dissolve the gelatin in 1 TBSP of water, set aside.

In a small saucepan, place raspberries and sugar, heat over low heat until sugar is dissolved and sauce thickens a bit, around 4 minutes (longer if using frozen berries). Remove from the heat, let it cool for 1 minute and add in gelatin mixture. Mix until gelatin is dissolved. If desired, pour the sauce through a fine-mesh strainer, pushing with the back of a spoon to remove the seeds. Let it cool slightly and pour over the cake. Refrigerate until firm. (Omit the gelatin if you want to make a sauce, and cook raspberry mixture about 8 minutes, or longer if it's too runny. Cool and serve over the cake.)