Preheat the oven to 350 degrees F and line an 8-inch square baking pan with parchment paper so that the excess hangs over the sides for easy removal.
For the crust, sift together the flour, sugar and salt so that there are no lumps, then add the softened butter and vanilla extract and mix until everything is well combined.
Using your fingers or a spoon, press the mixture into the lined dish. If it’s too sticky and hard to spread, pop it in the fridge for 5 or so minutes, just until it cools enough to smooth out.
Bake for 14 minutes, just until the edges begin to brown. Remove the crust from the oven and turn the oven down to 325 degrees F.
For the filling, combine the sugar and lemon zest in a medium saucepan and massage between your fingers until all the zest is well incorporated. This will help release the oils in the lemon zest and create a more lemony flavor.
Cube the cold butter and put it in a medium bowl. Set a fine mesh strainer over the top and set aside somewhere within reach. You’ll use this to strain the lemon curd later.
Add the eggs and egg yolks into the sugar mixture and mix. Once combined, slowly whisk in the lemon juice until everything is incorporated,
Put the saucepan on the stove over a medium low heat, swapping the whisk for a silicone spatula and stirring constantly to keep it from sticking or burning. The spatula will help reduce the amount of air that goes into the mixture. You want as smooth of a filling as possible.
Cook the mixture for 12 to 16 minutes, just until it reaches 160 degrees F.
Once the lemon mixture has finished cooking, quickly remove from heat and strain with a fine mesh strainer into the chilled, cubed butter that had been set aside, gently mixing until everything is well combined and smooth. This will stop the cooking process. Be careful not to mix too hard, as you don’t want to add air bubbles into the mixture.
Pour the lemon curd filling into the baked crust and tap the dish on the counter to remove any small air bubbles.
Put the bars in the oven and cook for 11 to 13 minutes. You want the center to jiggle like stiff jello. If it's too watery, it needs more time, and if puffed, bubbly or cracked, it's overcooked.
Remove from the oven and allow to cool at room temperature for at least 45 minutes. The curd will continue to set as it cools. Once done, put in the fridge to finish the chilling process for at least 3 hours or better still overnight. This will be the hardest part but it's worth it!
Remove the lemon bars from the dish by lifting the excess parchment paper and cut into squares using a knife warmed under hot water and wiped on a dish towel after each cut for smooth and even slices.
Sprinkle some powdered sugar over the top and serve the gluten free lemon bars.