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 rest of the 1 cup butter, ½ cup powdered sugar, and ¼ teaspoon kosher salt on high until well combined.
Add the 2 cups all-purpose flourand 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. Be sure to press down firmly, to really pack it in, and make sure it comes up the sides of the pan a little bit. This will help with shrinking dough and also ensure the curd doesn't seep down under the crust.
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, 1 ½ cups sugar, ¾ cup lemon juice, 2 tablespoons lemon zest, pinch kosher salt, and ½ teaspoon baking powderon medium-high speed until well combined and pale in color.
Add the ¼ cup all-purpose flourand 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 extra powdered sugar, lemon zest, blackberries, and/or mint leaves, if desired.
Notes
Storage: Store these in your refrigerator to keep them cool and firm. To freeze, I recommend flash freezing them, then wrapping in plastic wrap individually.
Use regular lemons: if you don't have Meyer Lemons, use regular lemons. You may will need a few more to get the right amount of juice, since they are smaller and less juicy. I recommend adding 1/4 cup more sugar to the curd filling as well.
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.