Preheat oven to 350 degrees F. Have ready an 8 inch metal pan. For convenience, I lined mine with non-stick foil.
Combine flour, powdered sugar and salt in a large mixing bowl or bowl of food processor. Cut in butter with pastry blender or use the food processor and pulse until crumbly. Press into prepared pan and bake for 18-20 minutes or until edges are lightly browned.
Combine cream cheese and sugar in a mixing bowl and beat until smooth. Beat in the evaporated milk, then stir in the lemon zest and lemon juice. Beat until smooth. Add the egg and beat just until smooth (do not overbeat after adding the egg). Stir in the flour. Alternately, you can do this in the food processor or in a blender.
Pour mixture over crust and bake for an additional 15 minute or until set. Cool in pan set on a wire rack. Chill until very cold. Spread with sour cream or sweetened whipped cream. If you’ve lined the pan with foil, you can now lift the bars from the pan, set on a cutting board and cut into bars or squares. Makes 12-16
Notes
Cook time also includes the chill time. It's just an estimate.