Whisk flour, sugar, and salt together in a metal bowl suitable for a double-boiler. Add melted butter and combine well. Push crust into the pan with your thumbs. Try to make it even and be sure to fill each corner. Bake until golden brown, about 20-25 minutes.
Meanwhile, whisk sugar, flour, cream of tartar, and salt together. (Use the bowl from the crust.) Thoroughly whisk in eggs and extra yolks. Add lemon zest and juice and combine well. Convert the bowl into a double-boiler; bring the water to a simmer. Whisk constantly while mixture heats. It will thicken in about 5-10 minutes.
Pour filling over baked crust and even it by tilting pan if needed. Bake until the filling is set, about 10-15 minutes. The center will often be a little lower than the edges; that's fine. It should have the consistency of gelatin. Let it cool, typically 2 hours. Pull the whole thing out via the foil and cut into bars. I usually do about 18 bars. Optionally dust with powdered sugar just before serving.