Preheat oven to 350 degrees. Line a 9x13 inch baking dish with parchment paper, leaving some extra to hang over the side of the baking dish to make it easy to transfer the squares after baking. In a large bowl, combine melted butter, flour and powdered sugar and mix together until combined, mixture should be thick and sticky. Pour dough into the baking dish and use your hands (or a spatula) to press down and even cover the bottom of the baking dish, forming a crust.
Bake for 25 minutes
While the crust is baking, you can make the filling. In a large bowl coming eggs, lemon juice, lemon zest, sugar, flour, baking powder and salt. Whisk everything together until thoroughly combined, mixture will be bright yellow, slightly thick but still runny. Once the crust is done baking pour the lemon filling right over the crust, make sure to cover the entire crust.
Return the pan to the oven to bake for 20 more minutes.
When done baking the bars will have a slightly pale yellow color on top - this is fine!
Allow the bars to cool completely and then refrigerate for at least an hour or so, so they can set. I like to refrigerate mine overnight.
When ready to serve, lift the bars out of the baking dish using the parchment paper and lightly sift powdered sugar over the top. Cut into squares and serve!