Using a blender, crush up graham crackers/Marie biscuits to fine crumbs. Alternatively, place crackers in a snap lock bag and beat with a rolling pin until fine.
Combine sugar, graham crackers and butter in a small bowl and mix together until combined.
Line a lightly buttered 8x8"/20x20cm square baking dish with parchment paper, leaving an overhang on one side (see images in post).
Place graham cracker base into the baking dish, and use fingertips or a measuring cup base to evenly and gently press the base into the dish. The base should be even in depth for an even cooking time.
Cheesecake
Preheat oven to 350F/180C
Beat cream cheese in a large bowl until smooth.
Add sugar, egg, lemon zest, lemon juice, sour cream and cornstarch and beat on medium for 5 minutes, until mixture is smooth and silky.
Pour cheesecake filling onto the base and use a spatula to smooth over the top to make sure you get an even cover.
Bake in the oven for 35 to 40 minutes. Cheesecake should be firm on the outside and slightly wobbly in the middle. The cheesecake should not be overly browned.
Cool cheesecake, then gently remove from baking dish onto a chopping board.
Cut into 9 equal pieces and serve with lemon wedges or whipped cream.