Place strips of parchment paper, cross-ways, to line the bottom, sides and over the edges of an 8-X8-inch baking pan.  Set aside. 

To Prepare Shortbread Crust:

Using a hand-held or standing mixer, cream butter and powdered sugar until combined well.  Add vanilla, all-purpose flour, and salt.  Mix on low until well combined.  Evenly press the dough into the bottom of the parchment lined baking pan.  Dock the top of the dough with a fork.  Place in the refrigerator for 30 minutes. 

Preheat oven to 325 degrees F. Bake shortbread crust for 25-30 minutes or until edges are lightly browned.  Let cool on a wire rack to room temperature. 

To Prepare Cheesecake Layer:

Using a hand-held or standing mixer, beat cream cheese and granulated sugar until fluffy.  Beat in the egg, lemon extract and lemon zest until smooth.  Cover and refrigerate until ready to assemble bars.

To Prepare Lemon Curd Layer:

In a medium saucepan, whisk together the egg yolks, cornstarch and granulated sugar.  Over low heat, whisk in water and lemon juice.  Slowly increase the heat to medium temperature, stirring constantly.  Cook until lemon mixture thickens to coat the back of a spoon. A finger run across the spoon back should leave a clean line in the curd.  Remove curd from heat.  Stir in butter and lemon zest.  Let cool.  

Assembling the Layers:

Spread the chilled cheesecake mixture evenly over the cooled shortbread crust.  Top the cheesecake layer evenly over the lemon curd mixture.  Bake for 35 to 40 minutes or until edges are a light golden brown.  Let cool to room temperature on a wire rack.  Chill for at least 1 hour in the refrigerator before cutting and serving.