½ cup fresh lemon juice (3 large or 4 medium lemons)
1 tablespoon finely grated lemon zest
Instructions
Preheat and prepare the pan: Preheat the oven to 350°F. Butter a 9 x 13-inch baking dish, then line it with parchment paper. Allow two edges to extend outside the pan to allow for easy removal.
Make the crust: Mix the flour, melted butter, powdered sugar, salt and vanilla extract in a bowl. Pour crumbles into the pan, spreading them out evenly with your fingers. Press them into the pan in an even layer, using the back of a measuring cup to pack it flat.
Bake the crust: Place the pan in the oven and bake 20 to 23 minutes until lightly golden brown. (You can bake the crust first and let it sit while making the filling, or mix up the filling while the crust bakes. In the second case, keep the eggs, dry ingredients, and lemon in separate bowls and whisk together once the crust is out of the oven).
Make the filling: Whisk the eggs, then whisk in the granulated sugar, flour, baking powder, lemon juice, and lemon zest until light and fluffy. Prick the baked crust gently with the tines of a fork and allow the crust to cool at least 5 minutes. Then pour the filling over the warm crust.
Bake the bars: Place the pan in the oven and bake for 10 minutes. Remove the pan and rotate it, then then bake 8 to 10 minutes more until lightly golden on top. It’s ok if it gets unevenly browned or if the top has small bubbles because it will be covered by the powdered sugar topping.
Chill: Allow the bars to cool to room temperature (about 1 hour), then transfer to the refrigerator for at least 2 to 3 hours to firm up (overnight is even better).
Top, slice and store: Sift powdered sugar over the top of the bars. Cut them into 24 squares, wiping off the knife after each cut to get a clean cut. Store bars refrigerated with parchment paper between the layers for up to 2 weeks. (Note that when sitting at room temperature, the powdered sugar topping can melt into the lemon filling, so you may need to refresh with new powdered sugar before serving leftovers.) You can also freeze lemon bars for up to 3 months without the powdered sugar topping, with parchment paper between the layers.