Prepare the crust
Preheat oven to 325°F (160°C).
Line an 8×8-inch (20×20 cm) baking pan with parchment paper.
Mix graham crumbs, sugar, and melted butter until combined.
Press firmly into the bottom of the pan.
Bake for 8–10 minutes, then let cool slightly.
Make the filling
Beat cream cheese until smooth and creamy.
Add sugar and mix until combined.
Beat in eggs one at a time.
Add sour cream, lemon juice, zest, and vanilla.
Mix until smooth (avoid overmixing).
Assemble & bake
Pour filling over the crust.
If using lemon curd, drop small spoonfuls on top and swirl with a knife.
Bake for 30–35 minutes, until center is slightly set but still soft.
Cool & chill
Let cool at room temperature for 1 hour.
Refrigerate for at least 3 hours (preferably overnight).
Slice into squares using a clean, sharp knife.