Line a 9x13-inch pan with parchment paper, leaving an overhang that you can use to lift the bars out later.
In a large bowl, combine flour, sugar and salt. Whisk together.
Cut butter into cubes and add to the flour mixture, along with the lemon extract. Use a food processor or pastry blender to cut the butter into the flour. You can also use your fingers. You want the mixture to resemble pea-sized crumbs.
Add the crumb mixture to the prepared dish and press it into an even layer with your fingers. Use the bottom of a measuring cup or glass to even out the crust.
Bake for 20-25 minutes, until slightly golden at the edges. It should have a dry appearance, not bubbly. Remove from oven.
While the crust is baking, prepare the curd. (Don’t make the curd ahead of time or it can curdle.)
In a medium bowl, beat together the eggs, yolk and sugar.
Add the flour and whisk until you no longer see clumps.
Stir in the lemon juice, lemon zest and vanilla extract.
Once the crust has finished baking, pour the curd mixture into the hot crust. Return the pan to the oven and bake for 10-12 minutes. The filling is ready when it no longer jiggles and has bubbles on top.
Remove the bars from the oven and place on a wire rack to cool for 1 hour, uncovered.
Use the parchment paper to lift the bars onto a cutting board. Cut ¼-inch off each side to create clean edges. Cut into squares or triangles.
Just before serving, dust with powdered sugar.
Notes
Tips & tricks
When pressing in the crumb mixture into the lined pan, use the bottom of a measuring cup so that it is even all around.The crust is done when it looks dry, not bubbly.Don’t make the lemon filling ahead of time because it can curdle before it’s baked into the base.You’ll know the lemon filling is ready when it no longer jiggles and has bubbles on top.When slicing the lemon bars, you want to make your lines as crisp as possible. So, slice off ⅛-inch of each side before cutting into squares or triangles.Remember that fresh lemon juice will always have a better flavor than bottled. Whenever possible, use freshly squeezed. A lemon squeezer makes this much easier.Adapted from Simply Recipes