In a food processor, finely chop the Kit Kat Bars. Spread evenly in the bottom of an 8x8 inch square pan. Bake for 5 minutes. Set aside.
MAKE THE FILLING:
In a large bowl, use an electric mixer to blend the cream cheese on high. Add the powdered sugar and egg and beat until smooth, scraping down the sides of the bowl. Add the lemon juice, lemon extract and lemon zest, and mix well. Pour the filling evenly over the Kit Kat crust.
ADD THE TOPPING:
In a medium bowl, use a fork to combine the flour, sugar, lemon extract, and softened butter until crumbly. Sprinkle the topping evenly over the lemon bar filling.
Bake for 18 to 20 minutes, or until the topping is lightly browned. Let cool completely; cut into bars.
Notes
Lemon extract can be found at Whole Foods and other specialty markets. The imitation stuff they sell in the regular grocery stores isn't so great.