Preheat your oven to 350 degrees. Spray a 9x13" baking pan with nonstick spray. Line the pan with parchment paper and spray again. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, cream butter and powdered sugar. This will kind of look like buttercream!
Add the salt and the flour. Mix until well combined. It should look like a dough
Pat the crust into an even layer in the prepared 9"x13" pan. Bake for 15 to 20 minutes. (Mine was perfect at 17).
To make filling, mix all of the filling ingredients in a mixing bowl and beat until a smooth, silky consistency. Once the crust is baked, pour the lemon filling over hot crust and bake again for 20 to 25 minutes. (Mine was perfect at 20 minutes).
After the bars have baked, let them cool completely. Sprinkle powdered sugar over the top of the bars and then cut into squares.