extra powdered sugar, lemon zest, blackberries, and/or mint leavesfor garnish (optional)
Liberally grease a 9x13 inch baking pan with a little bit of the softened butter. Preheat oven to 350 degrees F. Tip: If prepping and measuring all the other ingredients before starting, be sure to zest the lemons before juicing them!
In a large mixing bowl or stand mixer, mix the softened butter (1 cup/2 sticks), powdered sugar (1/2 cup), and kosher salt (1/4 teaspoon) on high until well combined.
Add the flour (2 cups) and mix on low speed until dough begins to just stick together and clump up (you should be able to form it into a shape easily with your hands, and you shouldn't see any visible flour).
Press the dough mixture evenly into the bottom of the prepared baking pan with your hands.
Bake at 350 for 20-25 minutes, or until just starting to brown.
Meanwhile, to make the topping: In the same mixing bowl, mix together the 5 eggs, sugar (1.5 cups), lemon juice (3/4 cup), lemon zest (2 tablespoons), a pinch of kosher salt, and baking powder (1/2 teaspoon) on medium-high speed until well combined and pale in color.
Add the flour (1/4 cup) and mix until smooth and not lumpy.
After crust has baked, remove from oven and turn the temperature down to 325 degrees F.
After the crust has cooled in the pan slightly (you should be able to touch the pan without a potholder), pour the topping in.
Bake at 325 degrees for 25-35 minutes, or until set (it shouldn't wiggle when you shake the pan gently).
Remove from oven and allow to cool in pan for at least 20 minutes, preferably longer - it will be much easier to cut if it's room temperature, so I like to plan to let it sit for about an hour.
Cut into squares and top with powdered sugar, blackberries, lemon zest, and/or mint leaves, if desired.
Storage: Store these in your refrigerator to keep them cool and firm.
Use regular lemons: if you don't have Meyer Lemons, use regular lemons. You will need a few more (around 8) to get the right amount of juice, since they are smaller and less juicy. You may want to add a little more sugar to the topping as well, since they are less sweet.
Some readers have commented that they take longer to bake/set. The original recipe called for baking them for 15-20 minutes after adding the filling. I cooked them again and found that they did take longer, so I adjusted the timing to 25-35 minutes. It may take longer or shorter for you, so keep an eye on them.