8 ounces unsalted butter, at room temperature
½ cup powdered sugar
2 cups all-purpose flour
¼ cup toasted walnuts
¼ teaspoon kosher salt
5 large eggs, at room temperature
2½ cups sugar
⅓ cup all-purpose flour
⅓ cup freshly squeezed lemon juice
2½ teaspoons freshly grated lemon zest
¾ teaspoon baking powder
Heat oven to 350 degrees.
For the crust: Place the butter in a mixing bowl and cream until light and fluffy, add the powdered sugar and beat again until light.
Place the flour, toasted walnuts and kosher salt in the bowl of a food processor and pulse several times until the walnuts are chopped very fine and incorporated with the flour, add this mixture to the butter and sugar and mix just until incorporated. Press this mixture into a 9-by-13-inch baking pan, making it even all around as you press it down. Bake for about 18 minutes until lightly browned.
For the filling: While the crust bakes, combine the eggs, sugar, flour, lemon juice, zest and baking powder in the mixing bowl, mix well on low or medium to thoroughly combine -- but don't whip on high or you will incorporate too much air.
When the crust has finished baking, pull out of the oven and immediately pour the filling over. Return carefully to the oven on the middle rack and bake 30-35 minutes until the filling is set and top is a deep golden brown.
Let cool completely on a rack, cut into bars, and sprinkle generously with powdered sugar when cool.
Cook's note: Don't let the crust cool at all before pouring in the filling and baking -- if it cools, the crust contracts and allows the filling to seep underneath making the edges very hard to cut and eat. You can leave out the walnuts in the crust and omit the step with the food processor. The nuts give it a nice added flavor.