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.
FILLING
In a large bowl with electric mixer, blend cream cheese on high. Add powdered sugar and egg and beat until smooth, scraping down the sides of the bowl. Add lemon juice, lemon extract and lemon zest, and mix well. Pour filling evenly over Kit Kat crust.
TOPPING
In a medium bowl, combine flour, sugar, lemon extract, and softened butter, and blend with a fork until crumbly. Sprinkle topping evenly over lemon bar filling.
Bake for 18 to 20 minutes, or until 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.