Preheat oven to 350 degrees. Line an 8x8 square pan with foil and lightly grease.
Make the Crust:
In a mixing bowl, stir together flour, powdered sugar and salt. Cut butter into small pieces and add it to the bowl along with the vanilla. Mix together to form a crumbly dough.
Press mixture firmly into the foil lined pan. Bake for 22 minutes.
Make the Lemon Layer:
Beat eggs and egg yolk together in a medium bowl. Add sugar, 2 Tablespoons flour, lemon juice and zest. Whisk for 2 minutes. Pour over hot crust. Return to oven for 25 minutes.
Cool completely before removing from pan in one large piece and cutting into squares.
Recipe by Crumbs and Chaos at http://www.crumbsandchaos.dreamhosters.com/2015/03/lemon-squares/