Preheat oven to 350.
Line a 9 x 9 inch pan with foil for ease in removing baked bars.
Crush cookies with rolling pin or food processor. Melt 3 tablespoons of the butter and mix with crushed cookies. Press down
in pan to form crust. Bake 8-10 minutes.
Whisk together the flour, baking soda, and salt. In a small bowl, add the sugar and lemon zest. Rub them together with your fingers until well mixed. This brings out the lemon flavor.
Beat 3 tablespoons of the butter until creamy. Add the sugar/lemon mixture and beat until pale and fluffy. Add the egg, and beat well to mix. Add the vanilla, and beat on low. Mix in the flour mixture and the buttermilk in batches at low speed, beginning and ending with the flour.