Preheat the oven to 350 degrees F. Prepare a 9 x 13” baking pan by greasing the bottom and sides and lining the bottom (lengthwise) with a long piece of parchment paper which extends over two ends of the pan to create handles.

In the large bowl of a mixer with the paddle attachment, cream butter and icing sugar at medium speed until fluffy. Reduce mixer speed to low, add flour, cornstarch and salt and blend until incorporated. The dough will bind into large clumps.

Drop dollops of dough onto the bottom of the prepared pan. With floured hands spread and press the dough firmly and evenly into the pan building it up about ¼” on all sides.

Bake crust for 20 to 22 minutes until lightly golden. Remove crust from oven and leave the oven on. The lemon filling (see below) should be poured in while the crust is still hot.

While the shortbread crust is baking, make the filling. In the large bowl of a mixer with the whisk attachment, beat eggs, sugar, lemon zest and lemon juice. Slowly add the flour and beat until the mixture becomes smooth.

Gently pour the filling over the hot crust. Carefully place the pan in the oven (the filling will be watery) and bake for 20 – 25 minutes until the centre is set, no longer jiggles, and the edges are lightly golden.

Remove from oven to a rack to cool at room temperature for about 2 hours. Then refrigerate the pan for at least 2 hours or ideally, overnight to ensure the bars are chilled completely.

Run a knife around the edges to release the bars from the sides of the pan. Using the parchment paper handles, gently lift the squares out of the pan to a cutting board. With a large chef’s knife, trim away about ¼” of the browned edges then cut into 32 squares. Dust squares with icing sugar. Serve the bars cold or at room temperature.