Crust: Preheat oven to 350°F. In a large mixing bowl with a paddle attachment, cream the butter and powdered sugar, add flour, and salt. Mix until it becomes uniformly crumbly. Line a 9×13 pan with foil or parchment paper. Pat the crust mixture into the lined pan evenly. Bake for 20 to 25 minutes or just until the edges turn a light golden brown, remove and set aside.
Filling: In a larger mixer bowl fitted with a whisk attachment, add eggs, sugar, lemon juice, zest and flour. Mix until smooth. Pour onto hot crust. Return to oven and bake for an additional 25-30 minutes. Remove and let cool to room temperature. Place in the refrigerator until set.
Remove from pan and cut into squares or triangles. Dust with powdered sugar. Store leftovers in the refrigerator until ready to use.mon