Preheat oven to 350°F (180°C) and grease a 20-by-20 cm pan.
Combine flour, granulated sugar and salt. Cut in butter until a rough crumbly texture (crust will remain crumbly and not come together). Press into prepared pan and bake for 18 minutes. Cool completely before filling.
For filling: Reduce oven temperature to 325°F (160°C). Beat cream cheese to soften, then whisk in sour cream. Whisk in granulated sugar, then eggs until smooth. Whisk in lemon juice and zest and pour over cooled crust.
Bake for 25 to 30 minutes, until filling no longer jiggles when pan is moved. Cool to room temperature, then cool for at least 4 hours before slicing. Cut into 36 squares. Lemon squares will keep up to 4 days refrigerated.
Per serving (2 squares): 3 g protein, 11 g fat (7 g saturated fat), 12 g carbohydrates, 0 g fibre, 65 mg cholesterol, 65 mg sodium.