Preheat oven to 350F. Line or spray a 9x9 pan. In a large bowl, whisk together flour, sugar, salt, baking powder and lemon zest. In a separate bowl or large liquid measuring cup, whisk together butter, eggs, egg yolk, lemon juice, and vanilla. Stir wet ingredients into flour mixture and mix until combined. Spread batter  into pan. Bake 18-20 minutes until a toothpick inserted in the center comes out clean. Don't overbake.

Lemon Glaze  Whisk together powdered sugar, lemon juice, and lemon zest until smooth and combined. When bars have cooled completely in pan, drizzle Glaze over top and use an offset spatula or knife to spread evenly over bars.

Chill bars 1-2 hours to let glaze set before cutting and removing from pan. May be served chilled (best) or at room temperature.