Preheat oven to 350 F degrees. Line an 8 x 8 inch baking pan with foil or parchment paper. Set aside.
In a medium sized mixing bowl, combine the crust ingredients: melted butter, flour, powdered sugar, and cornstarch. Press the shortbread mixture into the prepared pan and spread evenly. Bake for 10 minutes and remove from oven.
While the shortbread crust is baking, whisk together the filling ingredients: eggs, sugar, zest, lemon juice, flour, milk, and baking soda.
Pour the filling onto the baked shortbread crust and bake for an additional 25 minutes or until the edges are light golden and the filling is almost set.
Remove from oven and let cool completely at room temperature. Then cover with foil, and refrigerate for at least 2 hours or overnight.
When ready to serve, slice the bars into squares, and dust with powdered sugar and garnish with lemon slices if desired.