Preheat oven to 350°F (180°C). Grease an 8-in. (20 cm) square baking pan and line with two rectangles of parchment paper. The paper should hang over the edges of the pan to create handles.
Combine flour, walnuts, icing sugar, cornstarch and salt in a food processor and pulse until blended. Add cubes of butter and pulse until mixture resembles coarse crumbs. Press evenly into prepared pan and bake on the middle rack for 25 min., until golden brown. Remove and reduce oven temperature to 325°F (160°C).
Lightly whisk eggs, sugar, lemon juice, flour, vanilla and salt in a mixing bowl. Carefully pour custard over the warm crust and bake until set, about 25 min. Let cool to room temperature, then refrigerate at least 4 hours.
Lift out of the pan using the parchment paper handles, dust with icing sugar and cut into 16 pieces.
Cooking until "just set" means the edge of the square or cake should be firm, while the centre still jiggles slightly when the pan is nudged.