Start by crushing the digestive biscuits or graham crackers into fine crumbs. You can do this using a food processor, or place the biscuits in a plastic bag and crush them with a rolling pin.
In a large bowl, mix the crushed crumbs, melted butter, and sugar (if using). Stir until the mixture resembles wet sand.
Grease a 9-inch (23 cm) springform pan. Press the crumb mixture firmly into the bottom of the pan using the back of a spoon or glass.
Place the crust in the refrigerator while preparing the filling.
In a large bowl, beat the softened cream cheese until smooth and creamy.
Pour in the condensed milk and mix until fully combined.
Add the sour cream and vanilla extract. Mix until smooth.
Add eggs one at a time, mixing gently after each addition.
If using, mix in the lemon juice.
Pour the cheesecake filling into the prepared crust and smooth the top.
Preheat oven to 325°F (165°C).
Bake for 45–50 minutes, until the edges are set and the center is slightly jiggly.
Turn off the oven and leave the cheesecake inside for 1 hour.
Cool to room temperature, then refrigerate for at least 4 hours or overnight.