Preheat oven to 350 degrees F. Line a 13x9 inch metal pan with non-stick foil or parchment paper.
Combine the flour, powdered sugar and salt in a food processor and pulse to mix. Add the butter and pulse until mixture is coarse and crumbly. Measure out 1 cup of crumbs and set aside.
Pour remaining crumbs into prepared pan and press them down slightly (they’ll be very dry). Bake for 18 to 20 minutes.
Meanwhile, mix the filling. Whisk the eggs in a mixing bowl. Whisk in sugar, lemon juice, 4 tablespoons of flour and baking powder. Spread over top of baked crust. Sprinkle reserved cup of crumbs over top. Bake on center rack for 20 to 25 minutes.
Let cool completely. Lift from pan and set on a cutting board. Drizzle cooled bars with glazed (below). Let the glaze set then cut into 36 bars (or however many you want).
To make the icing, melt the butter in a microwave safe bowl. Add the sugar and stir until thick and pasty. Add the lemon juice 1 teaspoon at a time and stir until smooth and a drizzling consistency. You may drizzle it onto the squares or put it in a bag, snip off the corner and pipe it on.